Can I conditionally enforce a uniqueness constraint?
mysql, unique-constraint
Solution
Add another column called something like `isactive`. The create a unique constraint on `(username, isactive)`.
Then you can have both an active and inactive user name at the same time. You will not be able to have two active user names.
If you want multiple inactive names, use `NULL` for the value of `isactive`. `NULL` values can be repeated in a unique index.
Problem
My database contains a table of users. Every active user has a unique username. I'd like to be able to deactivate a user and free up the username they're using, but keep them in the same table. Is there a way to only conditionally enforce the uniqueness constraint?