The CONSTRAINT clause specifies a name for the table constraint.
    The name is used for a variety of purposes:

    o  The INTEG_FAIL error message specifies the name when an
       INSERT, UPDATE, or DELETE statement violates the constraint.

    o  The ALTER CONSTRAINT, DROP CONSTRAINT and ALTER TABLE DROP
       CONSTRAINT statements specify the constraint name.

    o  The SHOW TABLE statements display the names of constraints.

    o  The EVALUATING clause of the SET and the DECLARE TRANSACTION
       statements specifies constraint names.

    The CONSTRAINT clause is optional. If you omit the constraint
    name, SQL creates a name. However, Oracle Rdb recommends that
    you always name column and table constraints. The constraint
    names generated by SQL may be obscure. If you supply a constraint
    name with the CONSTRAINT clause, the name must be unique in the
    schema.