What is PRIMARY KEY Constraint in SQL Server?

Introduction:

In this article i will explain what is PRIMARY KEY Constraint in SQL Server.

Description:

In previous articles i explained what is Constraint, how many Constraints available in SQL Server, and what is NOT NULL, CHECK, and UNIQUE Constraint in SQL Server. Now i will explain what is PRIMARY KEY Constraint in SQL Server.

PRIMARY KEY Constraint:

PRIMARY KEY constraints identify the column or set of columns that have values that uniquely identify a row in a table. No two rows in a table can have the same primary key value. You cannot enter NULL for any column in a primary key.

Note:  A column or combination of columns that qualify as a primary key value is referred to as a candidate key.

We can create Primary Key constraint in two ways:

1. Column Level
2. Table Level

Column Level:

Now i'm creating Employee table where EmpNo column has Primary key constraint.

Create table Employee (    EmpNo int Primary key, -- Column Level    EmpName varchar(50) )

Table Level:

Now i'm creating Employee table where EmpNo column has Primary key constraint.

Create table Employee (    EmpNo int,    EmpName varchar(50),    constraint PK_EmpNo Primary Key(EmpNo) -- Table Level )

Difference between UNIQUE and PRIMARY KEY:

• UNIQUE allows only one null value. But where as Primary Key will not allow a nul value.
• Table can have only one primary key and can have any number of UNIQUE constraints.