Type constraints are different from check constraints. All databases provide type constraints, MySQL allowing you to turn them on or off as desired. Check constraints, however, are more comprehensive.

In this case, let's say you have a DECIMAL(23) for this 19 digit account number. What's preventing a 12 digit entry? In other words, the application will need to do a good amount of validation anyways. Anything the database provides in terms of type constraints is bonus. Check constraints, imho, are better implemented as a service wrapping the database connection or as a set of libraries. I personally don't like embedding constraints in the database because I feel that puts too much work into the datastore.