The first request adds a table-level Referential Constraint relationship on foreign key column a3 with column e1 in table e. Referential integrity is not enforced for this relationship.
The second request drops the foreign key Referential Constraint that was created by the first.
ALTER TABLE a ADD FOREIGN KEY (a3) REFERENCES WITH NO CHECK OPTION e(e1); ALTER TABLE a DROP FOREIGN KEY (a3) REFERENCES e(e1);