SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint is used to identify each row in a database table. It has some features:

• It must be unique value
• One table should have ONE primary key.
• One table should ONLY have one primary key.

To create a table with PRIMARY KEY:

SQL Server

CREATE TABLE Users
(
    UserID int NOT NULL PRIMARY KEY,
    FirstName varchar(100) NOT NULL,
    LastName varchar(100) NOT NULL 
)

My SQL

CREATE TABLE Users
(
    UserID int NOT NULL,
    FirstName varchar(100) NOT NULL,
    LastName varchar(100) NOT NULL,
    PRIMARY KEY (UserID)
)

To set PRIMARY KEY by using ALTER TABLE:

SQL Server / MySQL / Oracle / MS Access:

    ALTER TABLE Users
    ADD PRIMARY KEY (UserID)

A PRIMARY KEY can be a combine of multiple columns:
SQL Server / MySQL / Oracle / MS Access:

    ALTER TABLE Users
    ADD CONSTRAINT pk_UserName PRIMARY KEY (FirstName, LastName)

To drop a PRIMARY KEY:

SQL Server:

    ALTER TABLE Users
    DROP CONSTRAINT pk_UserName 

My SQL:

    ALTER TABLE Users
    DROP PRIMARY KEY