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For foreign keys defined to RESTRICT operations that would violate referential integrity, default checks occur at the time a statement executes. If you specify a CHECK ON COMMIT clause, then the checks occur only when the transaction is committed.
Setting the wait_for_commit database option controls the behavior when a foreign key is defined to restrict operations that would violate referential integrity. The CHECK ON COMMIT clause can override this option.
With the default wait_for_commit set to Off, operations that would leave the database inconsistent cannot execute. For example, an attempt to DELETE a department that still has employees in it is not allowed. The following statement gives an error:
DELETE FROM Departments WHERE DepartmentID = 200;
Setting wait_for_commit to On causes referential integrity to remain unchecked until a commit executes. If the database is in an inconsistent state, the database disallows the commit and reports an error. In this mode, a database user could drop a department with employees in it, however, the user cannot commit the change to the database until they:
Delete or reassign the employees belonging to that department.
Insert the DepartmentID row back into the Departments table.
Roll back the transaction to undo the DELETE operation.