SQL Primary Key

Primary Key In SQL

Primary key constraints does not allow to enter duplicate as well as null values. It always have unique data and it enforces the entity integrity of the table.Primary key doest not allow more than one coloumn in a table.

EMPLOYEE

E_ID	E_NAME	E_SALARY
201	Robert	37000
202	Michael	73500
203	Foster	45000
204	Luke	80000

Note: Here E_ID is the primary key in the Employee table.

Primary key on ALTER TABLE:

This way you can add unique key constrainton any coloumn of table in which table is already created.

Syntax:

ALTER TABLE EMPLOYEE
ADD primary key (E_Id);

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Primary Key In SQL

Primary key on ALTER TABLE:

This way you can add unique key constrainton any coloumn of table in which table is already created.