What is PRIMARY KEY

PRIMARY KEY

Primary key is cloumn in a table that uniquely identify the earch rows.

A table has only one primary key and primary can not be hold the null value. 

Following is the example to apply PRIMARY KEY Constraint on CREATE TABLE

CREATE TABLE Employee

(

Id INT NOT NULL,

Name varchar(255) NOT NULL,

Address varchar(255),

Age INT,

CONSTRAINT pk_EmployeeID PRIMARY KEY (Id)

)

Following is the example to apply PRIMARY KEY Constraint on ALTER TABLE

ALTER TABLE Employee

ADD PRIMARY KEY (Id)

Following is the example to DROP a PRIMARY KEY Constraint

ALTER TABLE Employee

DROP CONSTRAINT pk_EmployeeID

More SQL Constraints