Dbms ii mca-ch4-relational model-2013

Aruna Devi C (DSCASC)
1
Data Base Management System
[DBMS]
Chapter 4
Unit III
Relational Model

Relational Data Model
 The first commercial implementations of the relational model became
available in the early 1980s, such as the SQL/DS system on the MVS operating
system by IBM and the Oracle DBMS.
 Since then, the model has been implemented in a large number of commercial
systems.
 Current popular relational DBMSs (RDBMSs) include DB2 and Informix
Dynamic Server (from IBM), Oracle and Rdb (from Oracle), Sybase DBMS
(from Sybase) and SQLServer and Access (from Microsoft).
 In addition, several open source systems, such as MySQL and PostgreSQL, are
available.

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The relational model represents the database as a collection of relations.
The relational data model represents data in the form of the table.
Relational data consist of three components as follows:
 Data structure: Data are organized in the form of tables with rows and
columns.
 Data Manipulation: Powerful operations (using SQL) are used to
manipulate data, store data in the relation.
 Data integrity: Facilities are included to specify business rules that maintain
the integrity of data when they are manipulated.
Relational Model Concepts

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Relation/Table

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 Attribute is a column represent the attributes of a real-time object.
 When a relation is thought of a table of values, Each row represent a collection of
related data values.
 Tuple is a row, which represents a collection of related data values.
 Domains: A domain D is a set of atomic values.
 The set of allowed values for each attribute is called the domain of the Attribute.
 The data type describing the types of values that can appear in each column is
called a domain.
i.e., Domain represent set of all permissible values for that column.
Example: Ph-no. – Mobile or landline
Name
Emp Age – Value between 15 to 80
Tuples, Attribute, Domain, Relations
Relational Model Concepts

Relation:
 A relation schema R, denoted by R(A1, A2, ...,An), is made up of a relation name
R and a list of attributes, A1, A2, ..., An.
 Each attribute Ai is the name of a role played by some domain D in the relation
schema R.
 D is called the domain of Ai and is denoted by dom(Ai).
 A relation schema is used to describe a relation; R is called the name of this
relation.
Example:
STUDENT(Name: string, Ssn: string, Home_phone: string, Address: string,
Office_phone: string, Age: integer, Gpa: real)
Tuples, Attribute, Domain, Relations (Cont.,)

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 A1, A2, …, An are attributes
 R = (A1, A2, …, An ) is a relation schema
Example:
Customer_schema = (customer_name, customer_ street, customer_ city)
 r(R) is a relation on the relation schema R
Example: customer (Customer_schema)
An element t of r is a tuple, represented by a row in a table
Relation Schema
Jones
Smith
Curry
Lindsay
customer_name
Main
North
North
Park
customer_street
Harrison
Rye
Rye
Pittsfield
customer_city
customer
tuples
(or rows)
attributes
(or columns)

DEFINITION SUMMARY
Informal Terms Formal Terms
Table Relation
Column Attribute/Domain
Row Tuple
Values in a column Domain
Table Definition Schema of a Relation
Populated Table Extension

Relational Model Constraints
and Relational Database Schemas
Constraints on databases can generally be divided into three
main categories:
1. Constraints that are inherent in the data model. We call these inherent
model-based constraints or implicit constraints.
For example, the constraint that a relation cannot have duplicate tuples is an
inherent constraint.
2. Constraints that can be directly expressed in schemas of the data model,
typically by specifying them in the DDL (data definition language).
We call these schema-based constraints or explicit constraints.
3. Constraints that cannot be directly expressed in the schemas of the data model,
and hence must be expressed and enforced by the application programs.
We call these application-based or semantic constraints or business rules.

 Domain Constraint
 Key/Entity Constraint
 Referential Integrity Constraint
 Operational Constraint
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Types of Constraints:
and Relational Database Schemas (Cont.,)

Domain Constraint:
All the values that appear in a column of a relation must be taken from the
same domain. Domain means data type, size and the allowable range.
The data types include standard numeric data types for integers and real
numbers.
There are also characters, Booleans, fixed-length strings and variable length
strings, date, time, timestamp and so on.
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and Relational Database Schemas (Cont.,)

Key / Entity Constraint:
Key is a field that uniquely identifies the records, tables or data.
Different types of keys:
 Primary Key
 Foreign Key
 Unique key
 Super key
 Candidate key
 Primary Key:
A primary key is one or more columns in a table .
Every relation has a primary key, and the primary key values are
“unique and not null”.
Eg: Reg.No
Unique key :
Unique key is one or more columns in a table. It can contain Null value.
 Foreign Key:
Foreign key represent relationship between tables.
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Relational Model Constraints and Relational Database Schemas (Cont.,)

 Super key :
A super key is a set of one or more attributes that, taken collectively.
Eg: {Reg. no} or {Reg.No, Name}
 Candidate key :
A relation schema has more than one key, each of the keys is called a
candidate key.
All the key which satisfies the condition of primary key can be a candidate
key.
One of the candidate keys can be chosen to be the primary key.
Ex: {cname, SSN} is a super key,
but the attribute SSN is a candidate key.
{Reg.No} and {ph.No} are two candidate key in student table.
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Operational Constraint:
It is designed to enforce the business rules namely operational constraints
to the database.
Example: Min balance of Bank account is Rs. 1000/- ….etc.
Referential Integrity Constraint:
It is specified between two relations and is used to maintain the consistency
among tuples in the two relations.
Ensures that a value that appears in one relation for a given set of attributes
also appears for a certain set of attributes in another relation.
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Referential Integrity:
 Ensures that a value that appears in one relation for a given set of attributes also
appears for a certain set of attributes in another relation.
 Example: If “Perryridge” is a branch name appearing in one of the tuples in
the account relation, then there exists a tuple in the branch relation for
branch “Perryridge”.
 Primary and candidate keys and foreign keys can be specified as part of the SQL
create table statement:
 The primary key clause lists attributes that comprise the primary key.
 The foreign key clause lists the attributes that comprise the foreign key and
the name of the relation referenced by the foreign key. By default, a foreign
key references the primary key attributes of the referenced table.
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Constraints on a Single Relation
 Not Null
 Primary key
 Unique
 Check (P ), where P is a predicate
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Not Null Constraint
A constraint specifies whether null values are or are not permitted. The
constraint is specified as NOT NULL.
 Declaring branch_name for branch table as not null
branch_name char(15) not null
 Declaring the domain salary to be not null
salary numeric(12,2) not null

One possible database state for the COMPANY relational
database schema.

Database Schema for Company

Other Types of Constraints:
The preceding integrity constraints are included in the data definition
language (DDL).
Semantic Integrity Constraints:
 semantic integrity constraints, which may have to be specified and enforced on a
relational database.
Examples:
1. The salary of an employee should not exceed the salary of the employee’s
supervisor.
2. “The max. no. of hours per employee for all projects he or she works on is 56
hrs per week”.
- A constraint specification language may have to be used to express these
SQL allows triggers and ASSERTIONS to allow for some of these.
-----------------------------

Update Operations, Transactions,
and Dealing with Constraint Violations
 The operations of the relational model can be categorized into retrievals and
updates.
 A relational algebra expression forms a new relation after applying a number
of algebraic operators to an existing set of relations; its main use is for
querying a database to retrieve information.
 There are three basic operations that can change the states of relations in the
database:
Insert, Delete, and Update (or Modify)

The Insert Operation:
 The Insert operation provides a list of attribute values for a new tuple t that is to be inserted
into a relation R.
Insert can violate any of the four types of constraints:
 Domain constraints can be violated if an attribute value is given that does not appear in the
corresponding domain.
 Key constraints can be violated if a key value in the new tuple t already exists in another tuple
in the relation r(R).
 Entity integrity can be violated if any part of the primary key of the new tuple t is
NULL.
 Referential integrity can be violated if the value of any foreign key in t refers to a
tuple that does not exist in the referenced relation.
and Dealing with Constraint Violations (Cont.,)

The Insert Operation: Example
Insert <‘Cecilia’, ‘F’, ‘Kolonsky’, NULL, ‘1960-04-05’, ‘6357 Windy Lane,
Katy, TX’, F, 28000, NULL, 4> into EMPLOYEE.
 Result: This insertion violates the entity integrity constraint (NULL for the
primary key Ssn), so it is rejected.
Insert <‘Alicia’, ‘J’, ‘Zelaya’, ‘999887777’, ‘1960-04-05’, ‘6357 Windy Lane,
Katy, TX’, F, 28000, ‘987654321’, 4> into EMPLOYEE.
 Result: This insertion violates the key constraint because another tuple with
the same Ssn value already exists in the EMPLOYEE relation, and so it is
rejected.
Example

Example
The Insert Operation: Example (cont.,)
Insert <‘Cecilia’, ‘F’, ‘Kolonsky’, ‘677678989’, ‘1960-04-05’, ‘6357
Windswept, Katy, TX’, F, 28000, ‘987654321’, 7> into EMPLOYEE
 Result: This insertion violates the referential integrity constraint
specified on Dno in EMPLOYEE because no corresponding referenced
tuple exists in DEPARTMENT with Dnumber = 7.
Insert <‘Cecilia’, ‘F’, ‘Kolonsky’, ‘677678989’, ‘1960-04-05’, ‘6357 Windy
Lane, Katy, TX’, F, 28000, NULL, 4> into EMPLOYEE.
 Result: This insertion satisfies all constraints, so it is acceptable.

The Delete Operation:
 The Delete operation can violate only referential integrity.
 This occurs if the tuple being deleted is referenced by foreign keys from other
tuples in the database.
Example:
Delete the WORKS_ON tuple with Essn = ‘999887777’ and Pno = 10
 Result: This deletion is acceptable and deletes exactly one tuple.
Delete the EMPLOYEE tuple with Ssn = ‘999887777’
 Result: This deletion is not acceptable, because there are tuples in WORKS_ON
that refer to this tuple. Hence, if the tuple in EMPLOYEE is deleted, referential
integrity violations will result.
Company Database

The Update Operation:
The Update (or Modify) operation is used to change the values of one or
more attributes in a tuple (or tuples) of some relation R.
Example:
Update the salary of the EMPLOYEE tuple with Ssn = ‘999887777’ to 28000
 Result: Acceptable.
Update the Dno of the EMPLOYEE tuple with Ssn = ‘999887777’ to 1
 Result: Acceptable.
Update the Dno of the EMPLOYEE tuple with Ssn = ‘999887777’ to 7
 Result: Unacceptable, because it violates referential integrity.
Company Database

The Transaction Concept
 A transaction is an executing program that includes some database operations,
such as reading from the database, or applying insertions, deletions, or updates to
the database.
 For example, a transaction to apply a bank withdrawal will typically read the user
account record, check if there is a sufficient balance, and then update the record
by the withdrawal amount.

EXAMPLES

Aruna Devi.C (DSCASC) 31
branch (branch_name, branch_city, assets)
customer (customer_name, customer_street, customer_city)
loan (loan_number, branch_name, amount)
borrower (customer_name, loan_number)
account (account_number, branch_name, balance)
depositor (customer_name, account_number)
Database Schema for Bank

Aruna Devi.C (DSCASC) 32
Schema Diagram for a Banking Enterprise

Banking Enterprise

Dbms ii mca-ch4-relational model-2013

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