Er. Pradip Kharbuja

Topic 7 : SQL

SQL

Structured Query Language

SQL is a special-purpose programming language designed for managing data held in a Database Management System (DBMS).

Types of SQL Statements

1. Data Manipulation Language (DML)

2. Data Definition Language (DDL)

3. Data Control Language (DCL)

DML

DML is used for inserting, deleting and updating data in a database

DML is also used to retrieve and manipulate data in a database

DML modifies stored data not the database definition or database objects.

DML Statements are :

1. SELECT2. INSERT

3. UPDATE4. DELETE

5. COMMIT6. ROLLBACK

DDL

defines and controls the database structure

can add, change, or delete definitions of tables, fields and their data types

DDL Statements are :

1. CREATE

• table, database, index

2. ALTER

3. DROP

DCL

used to control access to data stored in a database.

DCL Statements

1. GRANT - to allow specified users to perform specified tasks like SELECT, INSERT, DELETE, CREATE, UPDATE, etc.

2. REVOKE - to cancel previously granted or denied permissions.

History of SQL

Developed from IBM’s SYSTEM R

It was developed as the standard database language by 1970s.

First standard published in 1987 by ISO based on work by ANSI

ISO - International Standards Organisation

ANSI – American National Standards Institute

History of SQL [Contd.]

Before 1987, SQL was developed by different vendors

Addition minor revisions in 1989

SQL: 1992 – Major revision ‘inSQL-92’

Introduced new data types like VARCHAR and new operators like UNION, JOIN and NATURAL JOIN

SQL: 1999 - with object-relational features

History of SQL [Contd.]

SQL: 2003 - introduced concept of ‘core SQL’

every implementation of core SQL gammar must meet.

main aim was to overcome the divergence in the language

SQL: 2006 – SQL to be used with XML

SQL: 2008 Minor revisions.

Data Manipulation Language : DML

For retrieving and updating data

SELECT – retrieving

INSERT, UPDATE, DELETE – updating

Difference between INSERT and UPDATE?

Literals

INSERT INTO workers (emp_no, first_name, last_name) VALUES (1, 'Ram', 'Shrestha');

Non-numeric in single quotes

Numeric NOT in quotes

INSERT

INSERT INTO workers (emp_no, first_name, last_name) VALUES (1, 'Ram', 'Shrestha');

INSERT INTO workers VALUES (1, 'Ram', 'Shrestha');

INSERT INTO workers (first_name, last_name) VALUES ('Lawerence', 'Smith');

UPDATE

UPDATE workers SET first_name = 'Emily';

UPDATE workers SET first_name = 'Emily' WHERE last_name = 'Smith';

UPDATE workers SET first_name = 'Emily' WHERE emp_no = 3;

DELETE

DELETE FROM workers;

DELETE FROM workers WHERE first_name = 'Lawerence';

DELETE FROM workers WHERE emp_no = 4;

Datatype

When we create a table or add a field to a table we specify the datatype.

Datatype is a classification that identifies the possible values for the field and operations that can be done on the data.

Datatype is also the way the data in that field is stored in the database.

Datatypes

Datatypes are much affected by different ‘flavours’ of SQL.

1. String

2. Number

3. Date

String Datatypes

1. CHAR - Fixed width character string. Maximum 8,000 characters

2. VARCHAR - Variable width character string. Maximum 8,000 characters

3. TEXT - Variable width character string. Maximum 2GB of text data

Char vs Varchar

Suppose 2 fields : one char 5 length another varchar 5 length

How these two will store 'Ram'?

In CHAR, Ram__

In VARCHAR, Ram

Numeric Datatypes

Integer, e.g. 8, -34, 0, 1000, -23

Numeric or Decimal, e.g. 8.23 with point set

Float or Real, e.g. 8.23 but could also be changed so that point moves when needed

Datetime Types

Date - System defined date

Time – System defind time

Timestamp – Dates and times including fractions of a second

Advantages of SQL

Universal

Easy to use

Fits (more or less) with relational model

Disadvantages of SQL

Does not support all features of relational model

No one standard and the different flavours in SQL

Has had to be extended

Much redundancy – possible to do the same thing many ways

ANY QUESTIONS?

Topic 07: SQL