To control the data that enters your database, you can apply powerful data-integrity rules called constraints. By applying a constraint to a column, you can prevent irreconcilable and empty data from being entered. R:BASE uses the following constraints:

·

Primary Key - A column or set of columns that uniquely identify a row; in other words, each value in a primary key column is unique. A primary-key constraint prevents duplicate (non-unique) and null values from being entered. Even if you do not specifically define a constraint, all tables (in a well-designed database) should have a primary key. You can define one primary key per table.

·

Unique Key - A column or set of columns that uniquely identify a row; in other words, each value in a unique-key column is unique. A unique-key constraint prevents duplicate (non-unique) and null values from being entered. The only difference between a unique key and a primary key is that you can define multiple unique keys per table.

·	Foreign Key - A column or set of columns that match values in a particular primary key or unique key defined in a different table. A value cannot be inserted or changed.

Constraints cannot be turned off and are always enforced; you must delete the constraint if you do not want it. However, because R:BASE works with constraints faster, use constraints instead of rules when possible.

You can remove constraints from columns. If you want to remove a primary- or unique-key constraint, you must first remove the foreign-key constraints that refer to it.

See also

ALTER TABLE Command

CREATE INDEX Command

SET FASTFK Command