Database - Primary Key

PRIMARY KEY

  The PRIMARY KEY constraint uniquely identifies each record

 in a database table.

Primary keys must contain unique values.

A primary key column cannot contain NULL values.

Each table should have a primary key, and each table can

have only ONE primary key.

COLUMN LEVEL CONSTRAINT

To add a primary key for a column.

To add a primary key while creating a table

 create table emp (empno int constraint pk1_emp primary key,

ename varchar(20),desg char(3),salary money,deptno int)

TABLE LEVEL CONSTRAINT

  To add a primary key for a table.

COMPOSITE PRIMARY KEY

    If we add primary key for more than one column it is called

 as composite primary key.

create table emp2(empno int ,ename varchar(20),desg char(3),

salary money,deptno int,constraint pk_emp2 primary

 key(empno,deptno))

Adding primary key for an existing table

     To add an primary key for an existing table,first me must alter the

column to not null(because primary does not allow null values).

Alter table emps add constraint Pk_emps primary key(empno)

Dropping a primary key

 alter table emps drop constraint pk_emps