They are easy to test for when data is entered.
create domain hourly-wage numeric(5,2)
constraint wage-value-test check(value >= 4.00)
Note that ``constraint wage-value-test'' is optional (to give a name to the test to signal which constraint is violated).
create domain account-number char(10)
constraint account-number-null-test check(value not null)
create domain account-type char(10)
constraint account-type-test check(value in (``Checking'', ``Saving''))