Deletes one or more constraint definitions. When the DELETE
    CONSTRAINT statement executes, Oracle Rdb deletes the constraint
    definition from the physical database. If you use the PATHNAME
    argument when you invoke the database, DELETE CONSTRAINT also
    deletes the constraint definition from the data dictionary.

    You cannot delete a constraint when there are active transactions
    that access the relation or relations involved.

    You must execute this statement in a read/write transaction.
    If there is no active transaction and you issue this statement,
    Oracle Rdb starts a read/write transaction implicitly.

    You cannot delete a constraint definition unless you have invoked
    the database that includes the constraint. Other users are
    allowed to be attached to the database when you issue the DELETE
    CONSTRAINT statement.

    Example:

    DELETE CONSTRAINT EMPLOYEE_ID_EXISTS.