Issue #13810: This should fix the DELETE statement for MySQL (UNTESTED!).

MySQL does not allow for SubSelects on the table you are updating or deleting from. So we have to use a statement with joins instead. Unfortunatly DeleteValues does not support the needed syntax, so we just execute the plain query.
Merge pull request #2056 from skywalker666/master

Issue #13810: This should fix the DELETE statement for MySQL (UNTESTED!).