Define a Primary Key Constraint

Define a primary key to enforce uniqueness for values entered in specified columns that do not allow nulls. If you define a primary key for a table in your database, you can relate that table to other tables, thus reducing the need for redundant data. A table can have only one primary key.

1. In the Database window, click Database Diagrams   under Objects, click the database diagram you want to open, and then click Design on the Database window toolbar.
2. In your database diagram, click the row selector for the database column you want to define as the primary key. If you want to select multiple columns, hold down the CTRL key while you click the row selectors for the other columns.
3. Right-click the row selector for the column and select Primary Key. A primary key index, named "PK_" followed by the table name, is automatically created; you can find it on the Indexes/Keys tab of the P roperties page.

   Warning   If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.

A primary key column is identified by a primary key symbol in its row selector.

If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique.

If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table in your database diagram. However, you can change the order of columns after the primary key is created. In the Column name grid, remove the columns from the primary key. Then add the columns back in the order you want. To remove a column from the key, simply remove the column name from the Column name list.