Topics: SQL, Data modeling, Data Definition Language Pages: 5 (1023 words) Published: February 11, 2014
Database Management System (DBMS):
It is a collection of interrelated data and set of programs to access the data. It contains information about a particular enterprise and also provides an environment that is both convenient and efficient to use.

Database Applications:
Banking: all transactions
Airlines: reservations, schedules
Universities: registration, grades
Sales: customers, products, purchases
Manufacturing: production, inventory, orders, supply chain
Human resources: employee records, salaries, tax deductions

Purpose of Database Systems:
In the early days, database applications were built on top of file systems. Separate files had to be maintained for different applications that could result in redundant data across the applications. The traditional file system can be illustrated as follows:

Drawbacks of using file systems to store data are as follows: 1. Data redundancy and inconsistency
Multiple file formats, duplication of information in different files 2. Difficulty in accessing data
Need to write a new program to carry out each new task
3. Data isolation — multiple files and formats
4. Integrity problems
Integrity constraints (e.g. account balance > 0) become part of program code Hard to add new constraints or change existing ones
5. 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 6. Concurrent access by multiple users

Concurrent access needed for performance
Uncontrolled concurrent accesses can lead to inconsistencies E.g. two people reading a balance and updating it at the same time 7. Security problems

Database systems offer solutions to many of the above problems. Database Approach:

Features of database approach:

Creates and maintains databases
Eliminates requirement for data definition statements
Acts as interface between application programs and physical data files Separates logical and physical views of data

Advantages of DBMS approach:

Control of data redundancy
Data consistency
More information from the same amount of data
Sharing of data
Improved data integrity
Improved security
Enforcement of standards
Economy of scale
Balance of confliction requirements
Improved data accessibility and responsiveness
Increased productivity
Improved maintenance through data independence
Increased concurrency
Improved backup and recovery services

Disadvantages of DBMS approach:

Cost of DBMS
Additional hardware costs
Cost of conversion
Higher impact of a failure

Components of DBMS:
Data definition language: Specifies content and structure of database and defines each data element Specification notation for defining the database schema E.g.
create table account (
account-number char(10),
balance integer)
DDL compiler generates a set of tables stored in a data dictionary
Data manipulation language: Manipulates data in a database It is a language for accessing and manipulating the data organized by the appropriate data model. DML also known as query language.

SQL is the most widely used query language.
Data dictionary: Stores definitions of data elements, and data characteristics

SQL: Structured Query Language, is the de facto language to communicate with the database. It is a combination of different languages: DDL: Data Definition Language – Create, Drop and Alter
DML – Data Manipulation Language – Insert, Delete, Update, Select TCL – Transaction Control Language – Commit, Rollback, Savepoint DCL – Data Control Language – Grant, Revoke

Degree of a relationship:
The degree of a relationship is the number of entities associated with the relationship. The n-ary relationship is the general form for degree n. Special cases are the binary, and ternary...
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