Constraints

Constraints let you define the way the Database Engine automatically enforces the integrity of a database. Constraints define rules regarding the values allowed in columns and are the standard mechanism for enforcing integrity.

1.       NOT NULL specifies that the column does not accept NULL values.

2.       CHECK constraints enforce domain integrity by limiting the values that can be put in a column.

3.       UNIQUE constraints enforce the uniqueness of the values in a set of columns.

4.       PRIMARY KEY constraints identify the column or set of columns that have values that uniquely identify         a row in a table.

5.       FOREIGN KEY constraints identify and enforce the relationships between tables.

 The ON DELETE clause controls what actions are taken when you try to delete a row to which existing foreign keys point. The ON DELETE clause has the following options:

·         NO ACTION specifies that the deletion fails with an error.

·         CASCADE specifies that all the rows with foreign keys pointing to the deleted row are also deleted.

·         SET NULL specifies that all rows with foreign keys pointing to the deleted row are set to NULL.

·         SET DEFAULT specifies that all rows with foreign keys pointing to the deleted row are set to their default value.