PRIMARY KEY Constraints

You can use PRIMARY KEY constraints to enforce entity integrity as well as referential integrity. A PRIMARY KEY constraint ensures that no duplicate values are entered, that NULL values are not allowed, and that an index is created to enhance performance.

    To create a PRIMARY KEY constraint

• In SQL Enterprise Manager's Server Manager window, double-click the appropriate table, choose the Advanced Features button, and then choose the Primary Key/Identity tab.

  (For details, search for "constraints" in SQL Enterprise Manager Help.)

OR

• Use the CREATE TABLE or ALTER TABLE statement.

  For example:

  ALTER TABLE authors
  ADD
  CONSTRAINT UPKCL_auidind PRIMARY KEY CLUSTERED (au_id)

  adds a PRIMARY KEY constraint on the au_id column. Note that this table already has a primary key defined on it.

  (For details, see the CREATE TABLE and ALTER TABLE statements in the Microsoft SQL Server Transact-SQL Reference.)