SQL Server: PRIMARY KEY Constraint

 SQL Server     sbo     Updated on Oct 24, 2013     VIKRAM     554   

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 NULLCHECK, 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.
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