Joe Celko's SQL for Smarties, 3rd Edition

14.4. IN() Predicate and Referential Constraints

One of the most popular uses for the IN() predicate is in a CHECK() clause on a table. The usual form is a list of values that are legal for a column, such as:

CREATE TABLE Addresses
(addressee_name CHAR(25) NOT NULL PRIMARY KEY,
 street_loc CHAR(25) NOT NULL,
 city_name CHAR(20) NOT NULL,
 state_code CHAR(2) NOT NULL
       CONSTRAINT valid_state_code
       CHECK (state_code IN ('AL', 'AK', ...)),
 ...);

This method works fine with a small list of values, but it has problems with a longer list. It is very important to arrange the values in the order that they are most likely to match to the two-letter state_code to speed up the search.

In Standard SQL a constraint can reference other tables, so you could write ...