RDB$RELATION_CONSTRAINTS stores the definitions of all table-level constraints: primary,
unique, foreign key, CHECK, NOT NULL constraints.
||The name of the table-level constraint defined by the user, or
otherwise automatically generated by the system
||The name of the constraint type: PRIMARY KEY, UNIQUE, FOREIGN KEY,
CHECK or NOT NULL
||The name of the table this constraint applies to
||Currently NO in all cases: Firebird does not yet support
||Currently NO in all cases
||The name of the index that supports this constraint. For a CHECK
or a NOT NULL constraint, it is NULL.