CS157A Lecture 2 DB Mangement Systems Prof. Sin-Min Lee Department of Computer Science San Jose State University
Purpose of Database Systems
View of Data
Data Definition Language
Data Manipulation Language
Overall System Structure
In this Lecture you will learn:
1. Database Theory
Why use database?
Data is a valuable corporate resource which needs adequate, accuracy, consistency and security controls.
The centralised control of data means that for many applications the data will already exist, and facilitate quicker development.
Data will no longer be related by application programs, but by the structure defined in the database.
Easier to maintain systems
First generation database systems
The network and hierarchical databases of the 1970’s
The first systems to offer DBMS function in a unified system
e.g. CODASYL, IMS
Second generation database systems
relational databases of the 1980’s
data independence and non procedural data manipulation language
e.g. DB2, INGRES, NON-STOP SQL, ORACLE, Rdb/VMS
Focused on business data processing
Third generation database systems
Problems with 2nd generation DBS
inadequate for a broader class of applications (than business data processing)
e.g. CAD, CASE, Hypertext,
storing text segments, graphics, etc is usually difficult in 2nd gen. systems
Does not support complex data (folders)
Most vendors are working on functional enhancements on their 2nd gen. systems
Surprising degree of consensus on these features
3rd gen systems includes the desired capabilities of next generation database systems
DATABASE AND DBMS CONCEPTS Database – is a collection of data, typically describing the activities of one or more related organizations. A Database Management System DBMS) is software designed to assist in maintaining and utilization large collections of data. The SQL (Structured Query Language), developed by IBM is now the standard query language.
ADVANTAGES OF A DBMS
Application programs should not, ideally, be exposed to details of data representation and storage.
Efficient Data access
A DBMS uses several powerful functions to store and retrieve data efficiently.
Data Integrity and Security
The DBMS enforces integrity constraints to get a kind of protection against prohibited access to data.
ADVANTAGES OF A DBMS
When any users share the data, centralizing the administration of data can offer significant improvements.
Concurrent Access and Crash Recovery
A DBMS schedules concurrent access to the data in such manner that users can think of the data as being accessed by only one user at a time. DBMS also protects users from the effects of system failures.
Reduced Application Development Time
DBMS includes several important functions that are common to many applications accessing data in the DBMS. In conjunction with the high-level interface to the data, facilitates quick application development.
DATA MODEL CONCEPT
A data model is a collection of high-level data description constructs that hide many low-level storage details.
Most database management systems today are based on the Relational data model .
THE RELATIONAL MODEL
The central data description construct is this model is a Relation, which can be thought of a set of records.
A Schema for a relation specifies:
Name of relation (e.g. Students)
Name of each field (or attribute or column)
Type of each field.
Schema Sudents (sid:string,age:integer)
Example of an Instance of Students Students Schema ------> Students (sid : String, Age : Integer) 35 53-688 28 53-666 Age Sid
Each row in the Students relation is a record that describes any student.
Integrity constraints are conditions that the records must satisfy. E.g. sid is unique.
Oracle uses relational( and also object) data model.
Levels of Abstraction in a DBMS
Conceptual Schema : or logical schema describes all relations that are stored in the database.
In the university example, these relations contain information about entities , such as students and faculty, and about relationships , such as students’ enrollment in courses.
It summarizes how the relations described in the conceptual schema are stored on secondary storage devices such as disks and tapes.
Creation of data structures called indexes , to speed up data retrieval operations
A sample physical schema for the university:
Store all relations as unsorted files of records
Create indexes on the first column of Students, Faculty, and Courses relations.
Each external schema consists of a collection of one or more views and relations from the conceptual schema.
A view is conceptually a relation, but the records in a view are not stored in the DBMS.
A user creates any view from data already stored.
For example: we might want to allow students to find out the names of faculty members teaching courses.
This is the view associated:
Courseinfo ( cid:string , fname:string)
A user can treat a view just like a relation and ask questions about the records in the view, even though the records in the view are not stored explicitly .
Database Management System (DBMS)
A software package such as Oracle or MS-Access.
Manages data and relationships in the database.
Creates a Data Dictionary to store Metadata – data about data.
Manages all day-to-day transactions.
Provides user with data independence at application level.
Transforms logical data requests to match physical data structures.
Secures access through passwords, restricted user access, and encryption.
Provides backup and recovery mechanisms.
Provides export and import utilities.
Allows sharing of data with locking capabilities.
In the the past as new applications were written they used existing files, or created a new file for their use. Sometimes several existing files need to be sorted and merged to obtain the new file.Thus, it is probable that several files will contain the same information stored in different ways. In other words, there will be redundant and possibly inconsistent data. Consider the files for an insurance company Traditional File Systems POLICY# POLICYHOLDER PREMIUMS data ADDRESS PREMIUM-PA PREMIUM-TOTAL POLICY# POLICYHOLDER AGENCY data ADDRESS AGENT-CODE RENEWAL-DATE RENEWAL-AMT
Applications were often considered in relative isolation.
Data that should have been together was not.
The potential for flexible enquiry and reporting was limited.
All validations were in the programs.
Procedures were required to for backup and recovery.
All programmers had access to all records.
There was limited concurrent access.
Traditional File Systems
Basic Definitions Database: A collection of related data Data: Known facts that can be recorded Schema: Some part of the real world about which data is stored in the database. Database Management System(DBMS): A software package to facilitate the creation and maintenance of a computerised database.
Degrees of Data Independence
Data Access Organisation
Physical Record Location
Logical Views (Local)
Virtual Data Items
· Virtual Records
Data Value Characteristics
Data Element Name Only
Logical vs Physical Data Independence
GLOBAL LOGICAL DATABASE DESCRIPTION Physical Files Logical Data Independence Physical Data Independence
Three Schema Architecture
ANSI & ISO suggest that DBMS should have three schemas
CONCEPTUAL SCHEMA - the global logical model of the data and processing of the enterprise. i.e community user view.
EXTERNAL SCHEMA(S) - the logical application views of the CS. i.e individual user views.
INTERNAL SCHEMA - internal level storage view.
Three Schema Architecture External Schema 1 External Schema 2 External Schema n Conceptual Schema Internal Schema
The External Level
Each user has a language through which they access or see the database.
For the programmer - COBOL etc, for the end-user a query language or special purpose language.
All languages will contain a data sub-language which may be tightly or loosely coupled to the host language.
DSLs generally contain a data definition language DDL and data manipulation language DML.
The Conceptual Level
A representation of the entire information content of the database.
Defined with a Conceptual Schema Language which does not represent any storage or access details.
Should include all security and integrity rules and some suggest the CS should describe the total enterprise including all allowable processing.
The Internal Level
Low level representation of entire database.
Deals with stored records rather than conceptual or external records.
Stored records may differ in structure from conceptual records and external records.
The Internal Schema is still one level away from physical records which are often called pages or blocks
Inter-Related Data CLAIMS RENEWALS AGENCY D B M S RENEWALS CLAIMS AGENCY QUERY Data related by structure Flexible enquiry easier
Multiple Applications CLAIMS AGENCY DATABASE LOCAL VIEWS RENEWALS
Database Management System (DBMS)
Collection of interrelated data
Set of programs to access the data
DBMS contains information about a particular enterprise
DBMS provides an environment that is both convenient and efficient to use.
Human resources: employee records, salaries, tax deductions
Databases touch all aspects of our lives
Purpose of Database System
In the early days, database applications were built on top of file systems
Drawbacks of using file systems to store data:
Data redundancy and inconsistency
Multiple file formats, duplication of information in different files
Difficulty in accessing data
Need to write a new program to carry out each new task
Data isolation — multiple files and formats
Integrity constraints (e.g. account balance > 0) become part of program code
Hard to add new constraints or change existing ones
Purpose of Database Systems (Cont.)
Drawbacks of using file systems (cont.)
Atomicity of updates
Failures may leave database in an inconsistent state with partial updates carried out
E.g. transfer of funds from one account to another should either complete or not happen at all
Concurrent access by multiple users
Concurrent accessed needed for performance
Uncontrolled concurrent accesses can lead to inconsistencies
E.g. two people reading a balance and updating it at the same time
Database systems offer solutions to all the above problems
Levels of Abstraction
Physical level describes how a record (e.g., customer) is stored.
Logical level: describes data stored in database, and the relationships among the data.
type customer = record name : string; street : string; city : integer; end ;
View level: application programs hide details of data types. Views can also hide information (e.g., salary) for security purposes.
View of Data An architecture for a database system
Instances and Schemas
Similar to types and variables in programming languages
Schema – the logical structure of the database
e.g., the database consists of information about a set of customers and accounts and the relationship between them)
Analogous to type information of a variable in a program
Physical schema : database design at the physical level
Logical schema : database design at the logical level
Instance – the actual content of the database at a particular point in time
Analogous to the value of a variable
Physical Data Independence – the ability to modify the physical schema without changing the logical schema
Applications depend on the logical schema
In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.
A collection of tools for describing
semi-structured data models
Older models: network model and hierarchical model
Example of schema in the entity-relationship model
Entity Relationship Model (Cont.)
E-R model of real world
E.g. customers, accounts, bank branch
Relationships between entities
E.g. Account A-101 is held by customer Johnson
Relationship set depositor associates customers with accounts
Widely used for database design
Database design in E-R model usually converted to design in the relational model (coming up next) which is used for storage and processing
Example of tabular data in the relational model
customer- name Customer-id customer- street customer- city account- number Johnson Smith Johnson Jones Smith 192-83-7465 019-28-3746 192-83-7465 321-12-3123 019-28-3746 Alma North Alma Main North Palo Alto Rye Palo Alto Harrison Rye A-101 A-215 A-201 A-217 A-201 Attributes
A Sample Relational Database
Data Definition Language (DDL)
Specification notation for defining the database schema
DDL compiler generates a set of tables stored in a data dictionary
Data dictionary contains metadata (i.e., data about data)
Data storage and definition language
language in which the storage structure and access methods used by the database system are specified
Usually an extension of the data definition language
Data Manipulation Language (DML)
Language for accessing and manipulating the data organized by the appropriate data model
DML also known as query language
Two classes of languages
Procedural – user specifies what data is required and how to get those data
Nonprocedural – user specifies what data is required without specifying how to get those data
SQL is the most widely used query language
SQL: widely used non-procedural language
E.g. find the name of the customer with customer-id 192-83-7465 select customer.customer-name from customer where customer.customer-id = ‘192-83-7465’
E.g. find the balances of all accounts held by the customer with customer-id 192-83-7465 select account.balance from depositor , account where depositor.customer-id = ‘192-83-7465’ and depositor.account-number = account.account-number
Application programs generally access databases through one of
Language extensions to allow embedded SQL
Application program interface (e.g. ODBC/JDBC) which allow SQL queries to be sent to a database
Users are differentiated by the way they expect to interact with the system
Application programmers – interact with system through DML calls
Sophisticated users – form requests in a database query language
Specialized users – write specialized database applications that do not fit into the traditional data processing framework
Naïve users – invoke one of the permanent application programs that have been written previously
E.g. people accessing database over the web, bank tellers, clerical staff
Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise’s information resources and needs.
Database administrator's duties include:
Storage structure and access method definition
Schema and physical organization modification
Granting user authority to access the database
Specifying integrity constraints
Acting as liaison with users
Monitoring performance and responding to changes in requirements
A transaction is a collection of operations that performs a single logical function in a database application
Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g., power failures and operating system crashes) and transaction failures.
Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.
Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.
The storage manager is responsible to the following tasks:
interaction with the file manager
efficient storing, retrieving and updating of data
Overall System Structure
Two-tier architecture : E.g. client programs using ODBC/JDBC to communicate with a database
Three-tier architecture : E.g. web-based applications, and applications built using “middleware”