Primary keys

The primary key of a table is the column whose values are different in every row. Because they are different, they make each row unique. If no one such column exists, the primary key is a composite of two or more columns whose values, taken together, are different in every row.

Every table in the model must have a primary key. This rule follows automatically from the rule that all rows must be unique. If necessary, the primary key is composed of all the columns taken together. You shouldn't use long character strings as primary keys.

NULL values are never allowed in a primary-key column. NULL values are not comparable; that is, they cannot be said to be alike or different. Hence, they cannot make a row unique from other rows. If a column permits NULL values, it cannot be part of a primary key. When you define a PRIMARY KEY constraint, the database server also silently creates a NOT NULL constraint on the same column, or on the same set of columns that make up the primary key.

Some entities have ready-made primary keys such as catalog codes or identity numbers, which are defined outside the model. Sometimes more than one column or group of columns can be used as the primary key. All columns or groups that qualify to be primary keys are called candidate keys. All candidate keys are worth noting because their property of uniqueness makes them predictable in a SELECT operation.