The DELETE rule defines the effects that deleting a row in the referenced table has on the referencing table (see referential constraint definition). The DELETE rule is also used when column attributes are defined.

• No DELETE rule: deleting a row in the referenced table will fail if matching rows exist.
• ON DELETE CASCADE: if a row in the referenced table is deleted, all of the matching rows are deleted.
• ON DELETE RESTRICT: deleting a row in the referenced table will fails if matching rows exist.
• ON DELETE SET DEFAULT: if a row in the referenced table is deleted, the associated DEFAULT value is assigned to each referencing column for each matching row.
  A DEFAULT specification must exist for each referencing column.
• ON DELETE SET NULL: if a row in the referenced table is deleted, a NULL value is assigned to each referencing column of every matching row.
  None of these referencing tables may be a NOT NULL column.