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Working with Constraints

Constraints limit or control the types of data that can be entered into your tables. There are seven main categories of constraints. They include primary key constraints, foreign key constraints, default constraints, not null constraints, check constraints, rules, and unique constraints. The text that follows covers each of these constraint types in detail.

Primary Key Constraints

A primary key constraint is a column or a set of columns that uniquely identify a row in the table. Although you can designate more than one field as the primary key, each table can have only one primary key.


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