A constraint defines a condition that restricts the values that
    can be stored in a table. When you insert and update column
    values, the constraint checks the values against the conditions
    specified by the constraint. If a value violates the constraint,
    SQL generates an error message and the statement fails (either
    when the INSERT, UPDATE, or DELETE statement executes, or
    when the next COMMIT statement executes depending on when SQL
    evaluates the constraint).

    You specify constraints in CREATE and ALTER TABLE statements.
    Optionally, you supply a name for the constraints following the
    CONSTRAINT keyword.