Calculating and Verifying Check Digits in T-SQL

A lot of numbers that we use everyday such as Bank Card numbers, Identification numbers, and ISBN codes, have check digits. As part of the routine data cleansing of such codes we must check that the code is valid- but do we? Dwain Camps shows how it can be done in SQL in such a way that it could even be used in a constraint, and keep bad data out of the database.