An apparatus, system, and storage medium that, in an embodiment, receive a constraint command that specifies a parent table, a primary key in the parent table, a child table, and a foreign key in the child table, and enforce that all values for the primary key in the parent table are present in the foreign key in the child table. In an embodiment, the enforcing may include receiving an insert command that specifies a target key, a target value for the target key, and a target table; determining whether the target table matches the parent table and whether, within a transaction that includes the insert command, the child table includes at least one row with a foreign key value that equals the target value of the primary key; inserting the target value in the target table if the determining is true; and returning an error otherwise.