Assign 1

A database is an integrated collection of logically related records or files which
consolidates records into a common pool of data records that provides data for many
applications. A database is a collection of information that is organized so that it can
easily be accessed, managed, and updated.

A Database Management System (DBMS) is a set of computer programs that controls
the creation, maintenance, and the use of the database of an organization and its end
users. It allows organizations to place control of organization-wide database development
in the hands of database administrators (DBAs) and other specialists. DBMSes may use
any of a variety of database models, such as the network model or relational model. In
large systems, a DBMS allows users and other software to store and retrieve data in a
structured way. It helps to specify the logical organization for a database and access and
use the information within a database. It provides facilities for controlling data access,
enforcing data integrity, managing concurrency controlled, restoring database.

Advantages of DBMS (Database Management Systems) are followings:
A true DBMS offers several advantages over file processing. The principal advantages
of a DBMS
are the followings:
• Flexibility: Because programs and data are independent, programs do not have to be
modified when types of unrelated data are added to or deleted from the database, or
when physical storage changes.

• Fast response to information requests: Because data are integrated into a single
database, complex requests can be handled much more rapidly then if the data were
located in separate, non-integrated files. In many businesses
, faster response means better customer service.

• Multiple access: Database software allows data to be accessed in a variety of
ways (such as through various key fields) and often, by using several programming
languages (both 3GL and nonprocedural 4GL programs).
• Lower user training costs: Users often find it easier to learn such systems and training
costs may be reduced. Also, the total time taken to process requests may be shorter,
which would increase user productivity.
• Less storage: Theoretically, all occurrences of data items need be stored only once,
thereby eliminating the storage of redundant data. System developers and database
designers often use data normalization to minimize data redundancy.

The DBMS has a number of advantages as compared to traditional computer file-based
processing approach. The DBA must keep in mind these benefits or capabilities during
designing databases, coordinating and monitoring the DBMS.

The main advantages of DBMS are described below.

Data Consistency:
By controlling the data redundancy, the data consistency is obtained. If a data item
appears only once, any update to its value has to be performed only once and the
updated value (new value of item) is immediately available to all users. If the DBMS has
controlled redundancy, the database system enforces consistency. It means that when
data item that appears more than once . In the database is updated, the DBMS
automatically updates each occurrence of a data item in the database. However some
database systems do not support enforce data consistency.

Integration of Data:
In DBMS, data in database is stored in tables. A single database contains multiple tables
and relationships can be created between tables (or associated data entities). This
makes easy to retrieve and update data.

Database Access Language
Many DBMSs provide SQL as database access language e.g., SQL to access data from
multiple tables of a database. Every DBMS has its own version of SQL.

Development of Application
The cost and time for developing new applications is also reduced. The DBMS provides
tools that can be used to develop application programs. For example, some wizards are
available to generate Forms and Reports. Stored procedures (stored on server side)
also reduce the size of application programs.

More Advantages:
Data sharing, Controlling Data Redundancy ,Data Security
, Data Atomicity, Creating Forms, Control Over Concurrency, Backup and Recovery
Procedures,Data Independence.
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Azi
answered 2 years ago
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There are some features of a database management system
which make it attractive for using a DBMS in preference to other systems. They are
given below:

They are Centralized data management, Data independence, and Systems integration.

In a database system, the DBMS manages the data and all access of data is through the
DBMS providing a key to effective data processing. It contrasts with conventional data
processing systems where each application program has a direct access to the data
when it reads or manipulates it.

In the conventional data processing application programs, the programs are usually
based on extensive knowledge of data structure and format. In such an environment any
change in data structure or format would entail appropriate changes to the application
programs. If major changes are to be made to the data, the application programs may
have to to be rewritten.

The database management system provides the interface between the application
programs and the data, in a database system. When there are changes made to the
data representation, the metadata maintained by the DBMS is altered but the DBMS
continues the process of providing data to application programs in the previous used
ways. The wherever it is necessary, DBMS handles the task of transforming of data.

The independence between programs and the data is called data independence. It is
important because every time some change is required to be made to the data structure.
The programs that were being used before the change would go on to work. For
providing a high degree of data independence, a DBMS must take account of a
sophisticated metadata management system.

In DBMS, all files are integrated into one system to reduce redundancies and make the
data management system more efficient. DBMS also provides centralized control of the
operational data. There are some of the advantages of data independence, integration
and centralized control:

Redundancies and inconsistencies can be reduced by using it. It provides better service
to the Users. Flexibility of the system is also improved.

Cost of system development and maintenance is lower than the other. Standards are
enforced. Security system and integrity can be improved. Project requirements can be
identified and data model are developed.
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Mdsrnzr226
Advantages of DBMS are given below.
(1) DBMS provides "Potential for Enforcing Standards". In large organization, database
approach allows a Database Administrator to define standards as well as gives
freedom of enforcing standards among database users. In a centralized database
environment, it is easy for a DBA to enforce standards than in those environments where
each user has control over his own files.

(2) DBMS Provides "Reduced Application Development Time". No doubt it takes more
time for designing a database than making a single application but once a database is
created then it becomes very easy to make new application by using facilities provided
by DBMS.
(3) DBMS provides flexibility. To the requirements we have to do changes in database.
Using DBMS, we can change the structure of database and advantage is that it will not
affect on the existing database.

(4) When a single user modifies the database then all the other users of database can
see this modification (update).
(5) DBMS provides multiple user interfaces.
(6) DBMS provides storage structure for Efficient Query Processing.
(7) DBMS provide a mechanism for controlling redundancy.
(8) DBMS provides backup and recovery.
(9) DBMS enforce integrity constraints.
(10) DBMS provides Persistent storage for program objects.
00

Honest
Security is important advantage of DBMS.
Next, We may see data in different view...
Example:
All the Bank Database can't see by the Pune. But some of the details about the
clients of the bank like name and addresses may see by the pune.
But entire database can view by the manager of the Bank.
This will done with the help of db.

Data Integrity
Functional Dependency....
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Karthikas
answered 2 months ago

1.4 ADVANTAGES OF USING DBMS

1.Controlling Redundancy
In traditional file processing, every user group maintains its own files.
Each group independently keeps files on their db e.g., students.
Therefore, much of the data is stored twice or more. Redundancy leads
to several problems :

duplication of effort
storage space wasted when the same data is stored repeatedly
files that represent the same data may become inconsistent (since the
updates are applied independently by each users group).
We can use controlled redundancy.

2.Restricting Unauthorized Access
A DBMS should provide a security and authorization subsystem.

Some db users will not be authorized to access all information in the db
(e.g., financial data).
Some users are allowed only to retrieve data.
Some users are allowed both to retrieve and to update database.

3.Providing Persistent Storage for Program Objects and Data Structures
Data structure provided by DBMS must be compatible with the programming
language’s data structures. E.g., Object oriented DBMS are compatible with
programming languages such as C++, SMALLTALK, and the DBMS
software automatically performs conversions between programming data
structure and file formats.

4.Permitting Inferencing and Actions Using Deduction Rules
Deductive database systems provide capabilities for defining deduction
rules for inferencing new information from the stored database facts.

5.Providing Multiple User Interfaces
(e.g., query languages, programming languages interfaces, forms, menu-
driven interfaces, etc.)
6.Representing Complex Relationships Among Data

7.Enforcing Integrity Constraints

8.Providing Backup and Recovery
(must provide facilities for recovering from hardware or software failures).
=------------------------------
Disadvantages of using a DBMS

A database system generally provides on-line access to the database for many users. In
contrast, a conventional system is often designed to meet a specific need and therefore
generally provides access to only a small number of users. Because of the larger number
of users accessing the data when a database is used, the enterprise may involve additional
risks as compared to a conventional data processing system in the following areas.

1. Confidentiality, privacy and security.
2. Data quality.
3. Data integrity.
4. Enterprise vulnerability may be higher.
5. The cost of using DBMS.

Confidentiality, Privacy and Security

When information is centralised and is made available to users from remote locations, the
possibilities of abuse are often more than in a conventional data processing system. To

reduce the chances of unauthorised users accessing sensitive information, it is necessary
to take technical, administrative and, possibly, legal measures.

Most databases store valuable information that must be protected against deliberate
trespass and destruction.

Data Quality

Since the database is accessible to users remotely, adequate controls are needed to control
users updating data and to control data quality. With increased number of users accessing
data directly, there are enormous opportunities for users to damage the data. Unless there
are suitable controls, the data quality may be compromised.

Data Integrity

Since a large number of users could be using a database concurrently, technical
safeguards are necessary to ensure that the data remain correct during operation. The
main threat to data integrity comes from several different users attempting to update the
same data at the same time. The database therefore needs to be protected against
inadvertent changes by the users.

Enterprise Vulnerability

Centralising all data of an enterprise in one database may mean that the database becomes
an indispensible resource. The survival of the enterprise may depend on reliable
information being available from its database. The enterprise therefore becomes
vulnerable to the destruction of the database or to unauthorised modification of the
database.

The Cost of using a DBMS

Conventional data processing systems are typically designed to run a number of well-
defined, preplanned processes. Such systems are often "tuned" to run efficiently for the
processes that they were designed for. Although the conventional systems are usually
fairly inflexible in that new applications may be difficult to implement and/or expensive
to run, they are usually very efficient for the applications they are designed for.

The database approach on the other hand provides a flexible alternative where new
applications can be developed relatively inexpensively. The flexible approach is not
without its costs and one of these costs is the additional cost of running applications that
the conventional system was designed for. Using standardised software is almost always
less machine efficient than specialised software.

Database - Advantages & Disadvantages

Advantages

• Reduced data redundancy
• Reduced updating errors and increased consistency
• Greater data integrity and independence from applications
programs
• Improved data access to users through use of host and query
languages
• Improved data security
• Reduced data entry, storage, and retrieval costs
• Facilitated development of new applications program

Disadvantages

• Database systems are complex, difficult, and time-consuming to
design
• Substantial hardware and software start-up costs
• Damage to database affects virtually all applications programs
• Extensive conversion costs in moving form a file-based system to
a database system

• Initial training required for all programmers and users

RDBMS Concepts
1. What is database?
A database is a logically coherent collection of data with some inherent meaning,

representing some aspect of real world and which is designed, built and populated

with data for a specific purpose.

2. What is DBMS?
It is a collection of programs that enables user to create and maintain a database.
In other words it is general-purpose software that provides the users with the processes of
defining, constructing and manipulating the database for various applications.

3. What is a Database system?
The database and DBMS software together is called as Database system.

4. Advantages of DBMS?

 Redundancy is controlled.
 Unauthorised access is restricted.
 Providing multiple user interfaces.
 Enforcing integrity constraints.
 Providing backup and recovery.

5. Disadvantage in File Processing System?
 Data redundancy & inconsistency.
 Difficult in accessing data.
 Data isolation.
 Data integrity.
 Concurrent access is not possible.
 Security Problems.

6. Describe the three levels of data abstraction?
The are three levels of abstraction:
 Physical level: The lowest level of abstraction describes how data are stored.
 Logical level: The next higher level of abstraction, describes what data are stored in
database and what relationship among those data.
 View level: The highest level of abstraction describes only part of entire database.
7. Define the "integrity rules"
There are two Integrity rules.
 Entity Integrity: States that “Primary key cannot have NULL value”
 Referential Integrity: States that “Foreign Key can be either a NULL value
or should be Primary Key value of other relation.

8. What is extension and intension?
Extension -
It is the number of tuples present in a table at any instance. This is time
dependent.
Intension -
It is a constant value that gives the name, structure of table and the
constraints laid on it.

9. What is System R? What are its two major subsystems?
System R was designed and developed over a period of 1974-79 at IBM San Jose
Research Center. It is a prototype and its purpose was to demonstrate that it is possible to
build a Relational System that can be used in a real life environment to solve real life
problems, with performance at least comparable to that of existing system.
Its two subsystems are
 Research Storage
 System Relational Data System.

10. How is the data structure of System R different from the relational structure?
Unlike Relational systems in System R
 Domains are not supported

 Enforcement of candidate key uniqueness is optional
 Enforcement of entity integrity is optional
 Referential integrity is not enforced

11. What is Data Independence?
Data independence means that “the application is independent of the storage
structure and access strategy of data”. In other words, The ability to modify the schema
definition in one level should not affect the schema definition in the next higher level.
Two types of Data Independence:
 Physical Data Independence: Modification in physical level should not
affect the logical level.
 Logical Data Independence: Modification in logical level should affect the
view level.
NOTE: Logical Data Independence is more difficult to achieve

12. What is a view? How it is related to data independence?
A view may be thought of as a virtual table, that is, a table that does not really
exist in its own right but is instead derived from one or more underlying base table. In
other words, there is no stored file that direct represents the view instead a definition of
view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view
can insulate users from the effects of restructuring and growth in the database. Hence
accounts for logical data independence.

13. What is Data Model?
A collection of conceptual tools for describing data, data relationships data
semantics and constraints.

14. What is E-R model?
This data model is based on real world that consists of basic objects called entities
and of relationship among these objects. Entities are described in a database by a set of
attributes.

15. What is Object Oriented model?
This model is based on collection of objects. An object contains values stored in
instance variables with in the object. An object also contains bodies of code that operate
on the object. These bodies of code are called methods. Objects that contain same types
of values and the same methods are grouped together into classes.

16. What is an Entity?
It is a 'thing' in the real world with an independent existence.

17. What is an Entity type?
It is a collection (set) of entities that have same attributes.

18. What is an Entity set?

It is a collection of all entities of particular entity type in the database.

19. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped together into an
entity set.

20. What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary key, and its
primary key compromises of its partial key and primary key of its parent entity, then it is
said to be Weak Entity set.

21. What is an attribute?
It is a particular property, which describes the entity.

22. What is a Relation Schema and a Relation?
A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name
R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r
be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of
n-values t=(v1,v2, ..., vn).

23. What is degree of a Relation?
It is the number of attribute of its relation schema.

24. What is Relationship?
It is an association among two or more entities.

25. What is Relationship set?
The collection (or set) of similar relationships.

26. What is Relationship type?
Relationship type defines a set of associations or a relationship set among a given
set of entity types.

27. What is degree of Relationship type?
It is the number of entity type participating.

25. What is DDL (Data Definition Language)?
A data base schema is specifies by a set of definitions expressed by a special
language called DDL.

26. What is VDL (View Definition Language)?
It specifies user views and their mappings to the conceptual schema.

27. What is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may specify the
mapping between two schemas.

28. What is Data Storage - Definition Language?
The storage structures and access methods used by database system are specified
by a set of definition in a special type of DDL called data storage-definition language.

29. What is DML (Data Manipulation Language)?
This language that enable user to access or manipulate data as organised by
appropriate data model.
 Procedural DML or Low level: DML requires a user to specify what data are needed
and how to get those data.
 Non-Procedural DML or High level: DML requires a user to specify what data are
needed without specifying how to get those data.

31. What is DML Compiler?
It translates DML statements in a query language into low-level instruction that
the query evaluation engine can understand.

32. What is Query evaluation engine?
It executes low-level instruction generated by compiler.

33. What is DDL Interpreter?
It interprets DDL statements and record them in tables containing metadata.

34. What is Record-at-a-time?
The Low level or Procedural DML can specify and retrieve each record from a set
of records. This retrieve of a record is said to be Record-at-a-time.

35. What is Set-at-a-time or Set-oriented?
The High level or Non-procedural DML can specify and retrieve many records in
a single DML statement. This retrieve of a record is said to be Set-at-a-time or Set-
oriented.

36. What is Relational Algebra?
It is procedural query language. It consists of a set of operations that take one or
two relations as input and produce a new relation.

37. What is Relational Calculus?
It is an applied predicate calculus specifically tailored for relational databases
proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.

38.How does Tuple-oriented relational calculus differ from domain-oriented relational
calculus
The tuple-oriented calculus uses a tuple variables i.e., variable whose only
permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables that range over the
underlying domains instead of over relation. E.g. ILL, DEDUCE.

39. What is normalization?
It is a process of analysing the given relation schemas based on their Functional
Dependencies (FDs) and primary key to achieve the properties
 Minimizing redundancy
 Minimizing insertion, deletion and update anomalies.

40. What is Functional Dependency?
A Functional dependency is denoted by X Y between two sets of attributes X
and Y that are subsets of R specifies a constraint on the possible tuple that can form a
relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X]
then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely
determines the value of component Y.

41. When is a functional dependency F said to be minimal?
 Every dependency in F has a single attribute for its right hand side.
 We cannot replace any dependency X A in F with a dependency Y A where Y is a
proper subset of X and still have a set of dependency that is equivalent to F.
 We cannot remove any dependency from F and still have set of dependency that is
equivalent to F.

42. What is Multivalued dependency?
Multivalued dependency denoted by X Y specified on relation schema R,
where X and Y are both subsets of R, specifies the following constraint on any relation r
of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also
exist in r with the following properties
 t3[x] = t4[X] = t1[X] = t2[X]
 t3[Y] = t1[Y] and t4[Y] = t2[Y]
 t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]

43. What is Lossless join property?
It guarantees that the spurious tuple generation does not occur with respect to
relation schemas after decomposition.

44. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple, indivisible) values.

45. What is Fully Functional dependency?
It is based on concept of full functional dependency. A functional dependency
X Y is full functional dependency if removal of any attribute A from X means that the
dependency does not hold any more.

46. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R
is fully functionally dependent on primary key.

47. What is 3NF?
A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the
following is true
 X is a Super-key of R.
 A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively dependent on
primary key.

48. What is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it is in 3NF and satisfies an additional
constraint that for every FD X A, X must be a candidate key.

49. What is 4NF?
A relation schema R is said to be in 4NF if for every Multivalued dependency
X Y that holds over R, one of following is true
 X is subset or equal to (or) XY = R.
 X is a super key.

50. What is 5NF?
A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ...,
Rn} that holds R, one the following is true
 Ri = R for some i.
 The join dependency is implied by the set of FD, over R in which the left side is key
of R.
51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies that should
hold on the the constraint can be enforced by simply enforcing the domain
constraint and key constraint on the relation.

52. What are partial, alternate,, artificial, compound and natural key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities and that are
related to same owner entity. It is sometime called as Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate
Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available, then the
last resort is to simply create a key, by assigning a unique number to each record or
occurrence. Then this is known as developing an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a
construct, then combining multiple elements to create a unique identifier for the construct
is known as creating a compound key.
Natural Key:

When one of the data elements stored within a construct is utilized as the
primary key, then it is called the natural key.

53. What is indexing and what are the different kinds of indexing?
Indexing is a technique for determining how quickly specific data can be found.
Types:
 Binary search style indexing
 B-Tree indexing
 Inverted list indexing
 Memory resident table
 Table indexing

54. What is system catalog or catalog relation? How is better known as?
A RDBMS maintains a description of all the data that it contains, information
about every relation and index that it contains. This information is stored in a collection
of relations maintained by the system called metadata. It is also called data dictionary.

55. What is meant by query optimization?
The phase that identifies an efficient execution plan for evaluating a query that
has the least estimated cost is referred to as query optimization.

56. What is join dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued dependency.A
JD {R1, R2, ..., Rn} is said to hold over a relation R if R1, R2, R3, ..., Rn is a lossless-
join decomposition of R . There is no set of sound and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns of
a relation are contained in other columns. A foreign key constraint is an example of
inclusion dependency.

57. What is durability in DBMS?
Once the DBMS informs the user that a transaction has successfully completed,
its effects should persist even if the system crashes before all its changes are reflected on
disk. This property is called durability.

58. What do you mean by atomicity and aggregation?
Atomicity:
Either all actions are carried out or none are. Users should not have to
worry about the effect of incomplete transactions. DBMS ensures this by undoing the
actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection of
entities and relationships. It is used when we need to express a relationship among
relationships.

59. What is a Phantom Deadlock?
In distributed deadlock detection, the delay in propagating local information
might cause the deadlock detection algorithms to identify deadlocks that do not really
exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.

60. What is a checkpoint and When does it occur?
A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the
DBMS can reduce the amount of work to be done during restart in the event of
subsequent crashes.

61. What are the different phases of transaction?
Different phases are
 Analysis phase
 Redo Phase
 Undo phase

62. What do you mean by flat file database?
It is a database in which there are no programs or user access languages. It has no
cross-file capabilities but is user-friendly and provides user-interface management.

63. What is "transparent DBMS"?
It is one, which keeps its Physical Structure hidden from user.

64. Brief theory of Network, Hierarchical schemas and their properties
Network schema uses a graph data structure to organize records example for such
a database management system is CTCG while a hierarchical schema uses a tree data
structure example for such a system is IMS.

65. What is a query?
A query with respect to DBMS relates to user commands that are used to interact
with a data base. The query language can be classified into data definition language and
data manipulation language.

66. What do you mean by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows to be used by
the parent query. Depending on how the subquery is written, it can be executed once for
the parent query or it can be executed once for each row returned by the parent query. If
the subquery is executed for each row of the parent, this is called a correlated subquery.
A correlated subquery can be easily identified if it contains any references to the
parent subquery columns in its WHERE clause. Columns from the subquery cannot be
referenced anywhere else in the parent query. The following example demonstrates a
non-correlated subquery.
E.g. Select * From CUST Where '10/03/1990' IN (Select ODATE From ORDER
Where CUST.CNUM = ORDER.CNUM)

67. What are the primitive operations common to all record management systems?

Addition, deletion and modification.

68. Name the buffer in which all the commands that are typed in are stored
‘Edit’ Buffer

69. What are the unary operations in Relational Algebra?
PROJECTION and SELECTION.

70. Are the resulting relations of PRODUCT and JOIN operation the same?
No.
PRODUCT: Concatenation of every row in one relation with every row in
another.
JOIN: Concatenation of rows from one relation and related rows from another.

71. What is RDBMS KERNEL?
Two important pieces of RDBMS architecture are the kernel, which is the
software, and the data dictionary, which consists of the system-level data structures used
by the kernel to manage the database
You might think of an RDBMS as an operating system (or set of subsystems),
designed specifically for controlling data access; its primary functions are storing,
retrieving, and securing data. An RDBMS maintains its own list of authorized users and
their associated privileges; manages memory caches and paging; controls locking for
concurrent resource usage; dispatches and schedules user requests; and manages space
usage within its table-space structures
.
72. Name the sub-systems of a RDBMS
I/O, Security, Language Processing, Process Control, Storage Management,
Logging and Recovery, Distribution Control, Transaction Control, Memory Management,
Lock Management

73. Which part of the RDBMS takes care of the data dictionary? How
Data dictionary is a set of tables and database objects that is stored in a special
area of the database and maintained exclusively by the kernel.

74. What is the job of the information stored in data-dictionary?
The information in the data dictionary validates the existence of the objects,
provides access to them, and maps the actual physical storage location.

75. Not only RDBMS takes care of locating data it also
determines an optimal access path to store or retrieve the data

76. How do you communicate with an RDBMS?
You communicate with an RDBMS using Structured Query Language (SQL)

77. Define SQL and state the differences between SQL and other conventional
programming Languages

SQL is a nonprocedural language that is designed specifically for data access
operations on normalized relational database structures. The primary difference between
SQL and other conventional programming languages is that SQL statements specify what
data operations should be performed rather than how to perform them.

78. Name the three major set of files on disk that compose a database in Oracle
There are three major sets of files on disk that compose a database. All the files are binary. These are

 Database files
 Control files
 Redo logs
The most important of these are the database files where the actual data resides.
The control files and the redo logs support the functioning of the architecture itself.
All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without
these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a
backup, if there is one.

79. What is an Oracle Instance?
The Oracle system processes, also known as Oracle background processes,
provide functions for the user processes—functions that would otherwise be done by the
user processes themselves
Oracle database-wide system memory is known as the SGA, the system global
area or shared global area. The data and control structures in the SGA are shareable, and
all the Oracle background processes and user processes can use them.
The combination of the SGA and the Oracle background processes is known as an
Oracle instance

80. What are the four Oracle system processes that must always be up and running for
the database to be useable
The four Oracle system processes that must always be up and running for the
database to be useable include DBWR (Database Writer), LGWR (Log Writer), SMON
(System Monitor), and PMON (Process Monitor).

81. What are database files, control files and log files. How many of these files should a
database have at least? Why?
Database Files
The database files hold the actual data and are typically the largest in size.
Depending on their sizes, the tables (and other objects) for all the user accounts can go in
one database file—but that's not an ideal situation because it does not make the database
structure very flexible for controlling access to storage for different users, putting the
database on different disk drives, or backing up and restoring just part of the database.
You must have at least one database file but usually, more than one files
are used. In terms of accessing and using the data in the tables and other objects, the
number (or location) of the files is immaterial.
The database files are fixed in size and never grow bigger than the size at
which they were created
Control Files

The control files and redo logs support the rest of the architecture. Any

database must have at least one control file, although you typically have more than

one to guard against loss. The control file records the name of the database, the date

and time it was created, the location of the database and redo logs, and the

synchronization information to ensure that all three sets of files are always in step.

Every time you add a new database or redo log file to the database, the information is

recorded in the control files.

Redo Logs
Any database must have at least two redo logs. These are the journals for
the database; the redo logs record all changes to the user objects or system objects. If any
type of failure occurs, the changes recorded in the redo logs can be used to bring the
database to a consistent state without losing any committed transactions. In the case of
non-data loss failure, Oracle can apply the information in the redo logs automatically
without intervention from the DBA.
The redo log files are fixed in size and never grow dynamically from the
size at which they were created.

82. What is ROWID?
The ROWID is a unique database-wide physical address for every row on every
table. Once assigned (when the row is first inserted into the database), it never changes
until the row is deleted or the table is dropped.
The ROWID consists of the following three components, the combination of
which uniquely identifies the physical storage location of the row.
 Oracle database file number, which contains the block with the rows
 Oracle block address, which contains the row
 The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of retrieving rows
with a particular key value. Application developers also use it in SQL statements as a
quick way to access a row once they know the ROWID

83. What is Oracle Block? Can two Oracle Blocks have the same address?
Oracle "formats" the database files into a number of Oracle blocks when they are
first created—making it easier for the RDBMS software to manage the files and easier to
read data into the memory areas.
The block size should be a multiple of the operating system block size. Regardless
of the block size, the entire block is not available for holding data; Oracle takes up some
space to manage the contents of the block. This block header has a minimum size, but it
can grow.

These Oracle blocks are the smallest unit of storage. Increasing the Oracle block
size can improve performance, but it should be done only when the database is first
created.
Each Oracle block is numbered sequentially for each database file starting at 1.
Two blocks can have the same block address if they are in different database files.

84. What is database Trigger?
A database trigger is a PL/SQL block that can defined to automatically execute
for insert, update, and delete statements against a table. The trigger can e defined to
execute once for the entire statement or once for every row that is inserted, updated, or
deleted. For any one table, there are twelve events for which you can define database
triggers. A database trigger can call database procedures that are also written in PL/SQL.

85. Name two utilities that Oracle provides, which are use for backup and recovery.
Along with the RDBMS software, Oracle provides two utilities that you can use
to back up and restore the database. These utilities are Export and Import.
The Export utility dumps the definitions and data for the specified part of the
database to an operating system binary file. The Import utility reads the file produced by
an export, recreates the definitions of objects, and inserts the data
If Export and Import are used as a means of backing up and recovering the
database, all the changes made to the database cannot be recovered since the export was
performed. The best you can do is recover the database to the time when the export was
last performed.

86. What are stored-procedures? And what are the advantages of using them.
Stored procedures are database objects that perform a user defined operation. A
stored procedure can have a set of compound SQL statements. A stored procedure
executes the SQL commands and returns the result to the client. Stored procedures are
used to reduce network traffic.

87. How are exceptions handled in PL/SQL? Give some of the internal exceptions' name
PL/SQL exception handling is a mechanism for dealing with run-time errors
encountered during procedure execution. Use of this mechanism enables execution to
continue if the error is not severe enough to cause procedure termination.
The exception handler must be defined within a subprogram specification. Errors
cause the program to raise an exception with a transfer of control to the exception-handler
block. After the exception handler executes, control returns to the block in which the
handler was defined. If there are no more executable statements in the block, control
returns to the caller.
User-Defined Exceptions
PL/SQL enables the user to define exception handlers in the declarations
area of subprogram specifications. User accomplishes this by naming an exception as in
the following example:
ot_failure EXCEPTION;
In this case, the exception name is ot_failure. Code associated with this handler is written
in the EXCEPTION specification area as follows:

EXCEPTION
when OT_FAILURE then
out_status_code := g_out_status_code;
out_msg := g_out_msg;
The following is an example of a subprogram exception:

EXCEPTION
when NO_DATA_FOUND then
g_out_status_code := 'FAIL';
RAISE ot_failure;
Within this exception is the RAISE statement that transfers control back to the ot_failure
exception handler. This technique of raising the exception is used to invoke all user-
defined exceptions.
System-Defined Exceptions
Exceptions internal to PL/SQL are raised automatically upon error.
NO_DATA_FOUND is a system-defined exception. Table below gives a complete list of
internal exceptions.

PL/SQL internal exceptions.

Exception Name Oracle Error
CURSOR_ALREADY_OPEN ORA-06511
DUP_VAL_ON_INDEX ORA-00001
INVALID_CURSOR ORA-01001
INVALID_NUMBER ORA-01722
LOGIN_DENIED ORA-01017
NO_DATA_FOUND ORA-01403
NOT_LOGGED_ON ORA-01012
PROGRAM_ERROR ORA-06501
STORAGE_ERROR ORA-06500
TIMEOUT_ON_RESOURCE ORA-00051
TOO_MANY_ROWS ORA-01422
TRANSACTION_BACKED_OUT ORA-00061
VALUE_ERROR ORA-06502
ZERO_DIVIDE ORA-01476

In addition to this list of exceptions, there is a catch-all exception named
OTHERS that traps all errors for which specific error handling has not been established.

88. Does PL/SQL support "overloading"? Explain
The concept of overloading in PL/SQL relates to the idea that you can define
procedures and functions with the same name. PL/SQL does not look only at the
referenced name, however, to resolve a procedure or function call. The count and data
types of formal parameters are also considered.
PL/SQL also attempts to resolve any procedure or function calls in locally defined packages before looking at globally
defined packages or internal functions. To further ensure calling the proper procedure, you can use the dot notation. Prefacing a
procedure or function name with the package name fully qualifies any procedure or function reference.

89. Tables derived from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes

(b) Are always in 1NF

90. Spurious tuples may occur due to
i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d) ii & iii

(a) i & iii because theta joins are joins made on keys that are not primary keys.

91. A B C is a set of attributes. The functional dependency is as follows
AB -> B
AC -> C
C -> B
a) is in 1NF
b) is in 2NF
c) is in 3NF
d) is in BCNF

(a) is in 1NF since (AC)+ = { A, B, C} hence AC is the primary key. Since C
B is a FD given, where neither C is a Key nor B is a prime attribute, this it is not in 3NF.
Further B is not functionally dependent on key AC thus it is not in 2NF. Thus the given
FDs is in 1NF.

92. In mapping of ERD to DFD
a) entities in ERD should correspond to an existing entity/store in DFD
b) entity in DFD is converted to attributes of an entity in ERD
c) relations in ERD has 1 to 1 correspondence to processes in DFD
d) relationships in ERD has 1 to 1 correspondence to flows in DFD

(a) entities in ERD should correspond to an existing entity/store in DFD

93. A dominant entity is the entity
a) on the N side in a 1 : N relationship
b) on the 1 side in a 1 : N relationship
c) on either side in a 1 : 1 relationship
d) nothing to do with 1 : 1 or 1 : N relationship

(b) on the 1 side in a 1 : N relationship

94. Select 'NORTH', CUSTOMER From CUST_DTLS Where REGION = 'N' Order By
CUSTOMER Union Select 'EAST', CUSTOMER From CUST_DTLS Where REGION
= 'E' Order By CUSTOMER
The above is
a) Not an error
b) Error - the string in single quotes 'NORTH' and 'SOUTH'
c) Error - the string should be in double quotes
d) Error - ORDER BY clause

(d) Error - the ORDER BY clause. Since ORDER BY clause cannot be used in
UNIONS

95. What is Storage Manager?
It is a program module that provides the interface between the low-level data
stored in database, application programs and queries submitted to the system.

96. What is Buffer Manager?
It is a program module, which is responsible for fetching data from disk storage
into main memory and deciding what data to be cache in memory.

97. What is Transaction Manager?
It is a program module, which ensures that database, remains in a consistent state
despite system failures and concurrent transaction execution proceeds without
conflicting.

98. What is File Manager?
It is a program module, which manages the allocation of space on disk storage
and data structure used to represent information stored on a disk.

99. What is Authorization and Integrity manager?
It is the program module, which tests for the satisfaction of integrity constraint
and checks the authority of user to access data.

100.What are stand-alone procedures?
Procedures that are not part of a package are known as stand-alone because they
independently defined. A good example of a stand-alone procedure is one written in a
SQL*Forms application. These types of procedures are not available for reference from
other Oracle tools. Another limitation of stand-alone procedures is that they are compiled
at run time, which slows execution.

101.What are cursors give different types of cursors.
PL/SQL uses cursors for all database information accesses statements. The
language supports the use two types of cursors
 Implicit
 Explicit

102.What is cold backup and hot backup (in case of Oracle)?
 Cold Backup:
It is copying the three sets of files (database files, redo logs, and
control file) when the instance is shut down. This is a straight file copy, usually from the
disk directly to tape. You must shut down the instance to guarantee a consistent copy.
If a cold backup is performed, the only option available in the event of
data file loss is restoring all the files from the latest backup. All work performed on the
database since the last backup is lost.
 Hot Backup:
Some sites (such as worldwide airline reservations systems) cannot
shut down the database while making a backup copy of the files. The cold backup is not
an available option.
So different means of backing up database must be used — the hot
backup. Issue a SQL command to indicate to Oracle, on a tablespace-by-tablespace basis,
that the files of the tablespace are to backed up. The users can continue to make full use
of the files, including making changes to the data. Once the user has indicated that he/she
wants to back up the tablespace files, he/she can use the operating system to copy those
files to the desired backup destination.
The database must be running in ARCHIVELOG mode for the hot
backup option.
If a data loss failure does occur, the lost database files can be restored
using the hot backup and the online and offline redo logs created since the backup was
done. The database is restored to the most consistent state without any loss of committed
transactions.

103.What are Armstrong rules? How do we say that they are complete and/or sound
The well-known inference rules for FDs
 Reflexive rule :
If Y is subset or equal to X then X Y.
 Augmentation rule:
If X Y then XZ YZ.
 Transitive rule:
If {X Y, Y Z} then X Z.
 Decomposition rule :
If X YZ then X Y.
 Union or Additive rule:
If {X Y, X Z} then X YZ.
 Pseudo Transitive rule :
If {X Y, WY Z} then WX Z.
Of these the first three are known as Amstrong Rules. They are sound because it
is enough if a set of FDs satisfy these three. They are called complete because using these
three rules we can generate the rest all inference rules.

104.How can you find the minimal key of relational schema?

Minimal key is one which can identify each tuple of the given relation schema
uniquely. For finding the minimal key it is required to find the closure that is the set of all
attributes that are dependent on any given set of attributes under the given set of
functional dependency.
Algo. I Determining X+, closure for X, given set of FDs F
1. Set X+ = X
2. Set Old X+ = X+
3. For each FD Y Z in F and if Y belongs to X + then add Z
to X+
4. Repeat steps 2 and 3 until Old X+ = X+

Algo.II Determining minimal K for relation schema R, given set of FDs F
1. Set K to R that is make K a set of all attributes in R
2. For each attribute A in K
a. Compute (K – A)+ with respect to F
b. If (K – A)+ = R then set K = (K – A)+

105.What do you understand by dependency preservation?
Given a relation R and a set of FDs F, dependency preservation states that
the closure of the union of the projection of F on each decomposed relation Ri is equal to
the closure of F. i.e.,
((ΠR1(F)) U … U (ΠRn(F)))+ = F+
if decomposition is not dependency preserving, then some dependency is lost in the
decomposition.

106.What is meant by Proactive, Retroactive and Simultaneous Update.
Proactive Update:
The updates that are applied to database before it becomes
effective in real world .
Retroactive Update:
The updates that are applied to database after it becomes effective
in real world .
Simulatneous Update:
The updates that are applied to database at the same time when it
becomes effective in real world .

107.What are the different types of JOIN operations?
Equi Join: This is the most common type of join which involves only
equality comparisions. The disadvantage in this type of join is that there

Unit-1 Database System Concept Lecture-2 Disadvantages of File processing
systems
Hi! The journey has started, today you will learn about the various flaws in the
conventional file processing systems. The actual reason for the introduction of DBMS.

Data are stored in files and database in all information systems. Files are collections of
similar records. Data storage is build around the corresponding application that uses the
files.
File Processing Systems
1• Where data are stored to individual files is a very old, but often used approach to
system development.
2• Each program (system) often had its own unique set of files.
Page No 1
Diagrammatic representation of conventional file systems
Unit-1 Database System Concept Lecture-2
Users of file processing systems are almost always at the mercy of the Information
Systems department to write programs that manipulate stored data and produce needed
information such as printed reports and screen displays.
What is a file, then?
A File is a collection of data about a single entity.
Files are typically designed to meet needs of a particular department or user group.
Files are also typically designed to be part of a particular computer application
Advantages:
11are relatively easy to design and implement since they are normally
based on a single application or information system.
22The processing speed is faster than other ways of storing data.

Disadvantages:
11Program-data dependence.
22Duplication of data.
33Limited data sharing.
44Lengthy program and system development time.
55Excessive program maintenance when the system changed.
66Duplication of data items in multiple files. Duplication can affect on
input, maintenance, storage and possibly data integrity problems.
77Inflexibility and non-scalability. Since the conventional files are
designed to support single application, the original file structure cannot
support the new requirements.
Page No 2
Today, the trend is in favor of replacing file-based systems and applications with
database systems and applications.

Database Approach
A database is more than a file – it contains information about more than one entity and
information about relationships among the entities.
Data about a single entity (e.g., Product, Customer, Customer Order, Department) are
each stored to a “table” in the database.
Databases are designed to meet the needs of multiple users and to be used in multiple
applications.
One significant development in the more user-friendly relational DBMS products is that
users can sometimes get their own answers from the stored data by learning to use data
querying methods.

Page No 3 Unit-1 Database System Concept Lecture-2
Advantages:
11Program-data independence.
22Minimal data redundancy, improved data consistency, enforcement of
standards, improved data quality.
33Improved data sharing, improved data accessibility and responsiveness.
44Increased productivity of application development.
55Reduced program maintenance Data can be shared by many
applications and systems.
66Data are stored in flexible formats. Data independence. If the data are
well designed, the user can access different combinations of same data for
query and report purposes.
77Reduce redundancy.

Database Application Size
Personal Computer Database:
1o Supports a single-user.
2o Stand-alone.
3o May purchase such an application from a vendor.
4o Can’t integrate data with other applications.

Workgroup Database:
1o Example would be a small team using the same set of applications such as
in a physician’s office.
2o Includes numerous workstations and a single server typically.

Department Database: Page No 4
1o A functional area (such as production) in a firm.
1o Same hardware/software as Workgroup database, but is specialized for the
department.

Enterprise Database:
1o Databases or set of databases to serve an entire organization.
2o May be distributed over several different physical locations.

3o Requires organizational standards for system development and
maintenance.

Figure 11.1 Conventional files versus the database
Database design in perspective
The focus is on the data from the perspective of the system designer. The output of
database design is database schema. Data models were developed during the definition
phase. Then, data structures supporting database technology are produced during the
database design.

Advantages of using a DBMS
There are three main features of a database management system that make it attractive to
use a DBMS in preference to more conventional software. These features are centralized
data management, data independence, and systems integration.
In a database system, the data is managed by the DBMS and all access to the data is
through the DBMS providing a key to effective data processing. This contrasts with
conventional data processing systems where each application program has direct access
to the data it reads or manipulates. In a conventional DP system, an organization is likely
to have several files of related data that are processed by several different application
programs.
In the conventional data processing application programs, the programs usually are based
on a considerable knowledge of data structure and format. In such environment any
change of data structure or format would require appropriate changes to the application
programs. These changes could be as small as the following:
11. Coding of some field is changed. For example, a null value that was coded as -1 is
now coded as -9999.
22. A new field is added to the records.
33. The length of one of the fields is changed. For example, the maximum number of
digits in a telephone number field or a postcode field needs to be changed.
44. The field on which the file is sorted is changed.
Page No 6
If some major changes were to be made to the data, the application programs may need to
be rewritten. In a database system, the database management system provides the
interface between the application programs and the data. When changes are made to the
data representation, the metadata maintained by the DBMS is changed but the DBMS
continues to provide data to application programs in the previously used way. The DBMS
handles the task of transformation of data wherever necessary.

This independence between the programs and the data is called data independence. Data
independence is important because every time some change needs to be made to the data
structure, the programs that were being used before the change would continue to work.
To provide a high degree of data independence, a DBMS must include a sophisticated
metadata management system.
In DBMS, all files are integrated into one system thus reducing redundancies and making
data management more efficient. In addition, DBMS provides centralized control of the
operational data. Some of the advantages of data independence, integration and
centralized control are:
Redundancies and inconsistencies can be reduced
In conventional data systems, an organization often builds a collection of application
programs often created by different programmers and requiring different components of

the operational data of the organization. The data in conventional data systems is often
not centralized. Some applications may require data to be combined from several
systems. These several systems could well have data that is redundant as well as
inconsistent (that is, different copies of the same data may have different values). Data
inconsistencies are often encountered in everyday life. For example, we have all come
across situations when a new address is communicated to an organization that we deal
with (e.g. a bank, or Telecom, or a gas company), we find that some of the
communications from that organization are received at the new address while others
continue to be mailed to the old address. Combining all the data in a database would
involve reduction in redundancy as well as inconsistency. It also is likely to reduce the
costs for collection, storage and updating of data.
Better service to the Users Page No 7
A DBMS is often used to provide better service to the users. In conventional systems,
availability of information is often poor since it normally is difficult to obtain information
that the existing systems were not designed for. Once several conventional systems are
combined to form one centralized data base, the availability of information and its up-to-
datedness is likely to improve since the data can now be shared and the DBMS makes it
easy to respond to unforeseen information requests.
Centralizing the data in a database also often means that users can obtain new and
combined information that would have been impossible to obtain otherwise. Also, use of
a DBMS should allow users that do not know programming to interact with the data more
easily.
The ability to quickly obtain new and combined information is becoming increasingly
important in an environment where various levels of governments are requiring
organizations to provide more and more information about their activities. An
organization running a conventional data processing system would require new programs
to be written (or the information compiled manually) to meet every new demand.
Flexibility of the system is improved
Changes are often necessary to the contents of data stored in any system. These changes
are more easily made in a database than in a conventional system in that these changes do
not need to have any impact on application programs.
Cost of developing and maintaining systems is lower
As noted earlier, it is much easier to respond to unforeseen requests when the data is
centralized in a database than when it is stored in conventional file systems. Although the
initial cost of setting up of a database can be large, one normally expects the overall cost
of setting up a database and developing and maintaining application programs to be lower
than for similar service using conventional systems since the productivity of
programmers can be substantially higher in using non-procedural languages that have
been developed with modern DBMS than using procedural languages.

Standards can be enforced
Since all access to the database must be through the DBMS, standards are easier to
enforce. Standards may relate to the naming of the data, the format of the data, the
structure of the data etc.
Security can be improved
In conventional systems, applications are developed in an ad hoc manner. Often different
system of an organization would access different components of the operational data. In
such an environment, enforcing security can be quite difficult.
Setting up of a database makes it easier to enforce security restrictions since the data is
now centralized. It is easier to control that has access to what parts of the database.
However, setting up a database can also make it easier for a determined person to breach
security. We will discuss this in the next section.
Integrity can be improved
Since the data of the organization using a database approach is centralized and would be
used by a number of users at a time, it is essential to enforce integrity controls.
Integrity may be compromised in many ways. For example, someone may make a
mistake in data input and the salary of a full-time employee may be input as $4,000 rather
than $40,000. A student may be shown to have borrowed books but has no enrolment.
Salary of a staff member in one department may be coming out of the budget of another
department. Page No 9
If a number of users are allowed to update the same data item at the same time, there is a
possibility that the result of the updates is not quite what was intended. For example, in
an airline DBMS we could have a situation where the number of bookings made is larger
than the capacity of the aircraft that is to be used for the flight. Controls therefore must be
introduced to prevent such errors to occur because of concurrent updating activities.
However, since all data is stored only once, it is often easier to maintain integrity than in
conventional systems.
Enterprise requirements can be identified
All enterprises have sections and departments and each of these units often consider the
work of their unit as the most important and therefore consider their needs as the most
important. Once a database has been set up with _entralized control, it will be necessary
to identify enterprise requirements and to balance the needs of competing units. It may
become necessary to ignore some requests for information if they conflict with higher
priority needs of the enterprise.
Data model must be developed
Perhaps the most important advantage of setting up a database system is the requirement
that an overall data model for the enterprise be built. In conventional systems, it is more
likely that files will be designed as needs of particular applications demand. The overall
view is often not considered. Building an overall view of the enterprise data, although
often an expensive exercise is usually very cost-effective in the long term.

DBMS ARCHITECTURE
We now discuss a conceptual framework for a DBMS. Several different frameworks have
been suggested over the last several years. For example, a framework may be developed
based on the functions that the various components of a DBMS must provide to its users.
It may also be based on different views of data that are possible within a DBMS. We
consider the latter approach. Page No 10
A commonly used views of data approach is the three-level architecture suggested by
ANSI/SPARC (American National Standards Institute/Standards Planning and
Requirements Committee). ANSI/SPARC produced an interim report in 1972 followed
by a final report in 1977. The reports proposed an architectural framework for databases.
Under this approach, a database is considered as containing data about an enterprise. The
three levels of the architecture are three different views of the data:
11. External – individual user view
22. Conceptual – community user view
33. Internal – physical or storage view

The three level database architecture allows a clear separation of the information meaning
(conceptual view) from the external data representation and from the physical data
structure layout. A database system that is able to separate the three different views of
data is likely to be flexible and adaptable. This flexibility and adaptability is data
independence that we have discussed earlier.
We now briefly discuss the three different views.
The external level is the view that the individual user of the database has. This view is
often a restricted view of the database and the same database may provide a number of
different views for different classes of users. In general, the end users and even the
applications programmers are only interested in a subset of the database. For example, a
department head may only be interested in the departmental finances and student
enrolments but not the library information. The librarian would not be expected to have
any interest in the information about academic staff. The payroll office would have no
interest in student enrolments. Page No 11
The conceptual view is the information model of the enterprise and contains the view of
the whole enterprise without any concern for the physical implementation. This view is
normally more stable than the other two views. In a database, it may be desirable to
change the internal view to improve performance while there has been no change in the
conceptual view of the database. The conceptual view is the overall community view of
the database and it includes all the information that is going to be represented in the
database. The conceptual view is defined by the conceptual schema which includes
definitions of each of the various types of data.

The internal view is the view about the actual physical storage of data. It tells us what
data is stored in the database and how. At least the following aspects are considered at
this level:
11. Storage allocation e.g. B-trees, hashing etc.
22. Access paths e.g. specification of primary and secondary keys, indexes and
pointers and sequencing.
33. Miscellaneous e.g. data compression and encryption techniques, optimization of
the internal structures.

Efficiency considerations are the most important at this level and the data structures are
chosen to provide an efficient database. The internal view does not deal with the physical
devices directly. Instead it views a physical device as a collection of physical pages and
allocates space in terms of logical pages.
The separation of the conceptual view from the internal view enables us to provide a
logical description of the database without the need to specify physical structures. This is
often called physical data independence. Separating the external views from the
conceptual view enables us to change the conceptual view without affecting the external
views. This separation is sometimes called logical data independence.
Assuming the three level view of the database, a number of mappings are needed to
enable the users working with one of the external views. For example, the payroll office
may have an external view of the database that consists of the following information
only:
11. Staff number, name and address.
22. Staff tax information e.g. number of dependents.
33. Staff bank information where salary is deposited.
44. Staff employment status, salary level, leaves information etc.
Page No 12
The conceptual view of the database may contain academic staff, general staff, casual
staff etc. A mapping will need to be created where all the staff in the different categories
are combined into one category for the payroll office. The conceptual view would include
information about each staff’s position, the date employment started, full-time or part-
time, etc. This will need to be mapped to the salary level for the salary office. Also, if
there is some change in the conceptual view, the external view can stay the same if the
mapping is changed. Page No 13

Summary
Now we are coming to the end of this lecture, but before parting we will revise
thethings. Files are collections of similar records. Data storage is build around

thecorresponding application that uses the files. Duplication of data items in
multiplefiles. Duplication can affect on input, maintenance, storage and possibly data
integrityproblems. Inflexibility and non-scalability. Since the conventional files are
designed tosupport single application, the original file structure cannot support the
newrequirements.
Today, the trend is in favor of replacing file-based systems and applications
withdatabase systems and applications.
Questions
11. Explain a file with its advantages and disadvantages
22. Define centralized data management, data independence and systems integration
33. Explain DBMS architecture
44. Explain the 3 different views or architecture of the data

Selected Bibliography

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Data dictionary:
"centralized repository of information about data such as meaning, relationships to other
data, origin, usage, and format."

A data dictionary is a reserved space within a database which is used to store information
about the database itself.

A data dictionary may contain
information such as:

• Database design information
• Stored SQL procedures
• User permissions
• User statistics
• Database process information
• Database growth statistics
• Database performance statistics

Data dictionaries do not contain any actual data from the database, only bookkeeping information for
managing it. Without a data dictionary, however, a database management system cannot access data from
the database.

database: A collection of related information stored in a structured format.
Database is often used interchangeably with the term table (Lotus Approach, for
instance, uses the term database instead of table). Technically, they're different: A
table is a single store of related information; a database can consist of one or more
tables of information that are related in some way. For instance, you could track all
the information about the students in a school in a students table. If you then
created separate tables containing details about teachers, classes and classrooms,
you could combine all four tables into a timetabling database. Such a multi-table
database is called a relational database.

data entry: The process of getting information into a database, usually done by
people typing it in by way of data-entry forms designed to simplify the process.

dbms: Database management system. A program which lets you manage
information in databases. Lotus Approach, Microsoft Access and FileMaker Pro, for
example, are all DBMSs, although the term is often shortened to 'database'. So, the
same term is used to apply to the program you use to organise your data and the
actual data structure you create with that program.

One of the most important parts of an Oracle database is its data dictionary,
which is a read-only set of tables that provides information about the database.
A data dictionary contains:

• The definitions of all schema objects in the database (tables, views,
indexes, clusters, synonyms, sequences, procedures, functions, packages,
triggers, and so on)
• How much space has been allocated for, and is currently used by, the
schema objects
• Default values for columns
• Integrity constraint information
• The names of Oracle Database users
• Privileges and roles each user has been granted
• Auditing information, such as who has accessed or updated various
schema objects
• Other general database information

The data dictionary is structured in tables and views, just like other database
data. All the data dictionary tables and views for a given database are stored in
that database's SYSTEM tablespace.

Not only is the data dictionary central to every Oracle database, it is an
important tool for all users, from end users to application designers and database
administrators. Use SQL statements to access the data dictionary. Because the
data dictionary is read only, you can issue only queries (SELECT statements)
against it's tables and views.

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