htmanning:

Since you haven't said, I'll go ahead and assume you've got an SQL database.

For that case, I'd suggest using some constraints to prevent people from checking in a key that's already checked in, or checking a key out that's already checked out.

If you can provide some detail on your database, I might be able to help a little.

...roboticus

When your only tool is a hammer, all problems look like your thumb.