SQL Primary Key constraints

The primary key is used to uniquely identify each record in the table. Primary key contain unique value .The primary key cannot contain NULL value, each table contain only one primary key.

Primary key on create table –

Create table employee

Id int NOT NULL,

First name varchar(250) NOT NULL,

Last name varchar(250),

City varchar(250),

Salary int,

CONSTRAINT emp_id PRIMARY KEY (ID, First Name)

Primary key on alter table-

ALTER table employee

Add primary key(Id)

Drop a Primary Key –

ALTER table employee

Drop CONSTRAINT emp_id

                                                             Author - Sachin Pathak