Primary Keys

A primary key is the primary identifier for a table, and is attached to one or more columns whose values uniquely identify every row in the table. Every table must have a primary key, composed of one or more of its columns.

In the following example, Employee number is the primary key for the table Employee, and so each employee must have one unique employee number.

You can define one or more columns as the primary key of a table from the list of columns.

1. Open the property sheet of a table and click the Columns tab, which lists all the columns defined for the table.
2. Select the check box in the P column for one or more columns in the list.
3. [optional] Click the Keys tab and rename the key or select it and click the Properties tool to open its property sheet.
4. Click OK.

• Rebuilding Primary Keys
  Rebuilding primary keys in a physical diagram updates primary keys for tables by creating primary keys for tables that have no key and a single unique index. Rebuilding primary keys is useful when not all of the primary keys could be reverse engineered from a database, or if you did not select the rebuild option for primary keys during reverse engineering.