SQL Constraints- Primary key

nThe PRIMARY KEY constraint uniquely identifies each record in a database table.

nPrimary keys must contain unique values.

nA primary key column cannot contain NULL values.

nEach table should have a primary key, and each table can have only ONE primary key.

Create-primary key

CREATE TABLE EMPtbl1( (EMPID int NOT NULL, EMPname varchar(255) NOT NULL,
company varchar(255),salary varchar(255),City varchar(255),CONSTRAINT pk_EMPtbl1 PRIMARY KEY (EMPID, salary))

Alter-primary key

ALTER TABLE EMPtbl1

ADD CONSTRAINT pk_EMPtbl1 PRIMARY KEY (EMPID, salary)

Drop-primary key

ALTER TABLE EMPtbl1

DROP CONSTRAINT pk_EMPtbl1