We’ve all done it.
Either we learn some tricks in MYSQL, or read a cool post about some obscure programming trick you can do with SQL and decide to try it in a pinch on a live database. Just don’t do it.
9 times out of 10 you have access to SSH, and PHP, and you can write a script that will do what you want. You can also “test” it on a small set of inputs and have it echo out the necessary queries to the console instead of sending them to MYSQL.
This is called the poor man’s unit test, or poor man’s BDD.
Now, on one word press site, before the whole wordpress MU thing, we made the mistake of using qTranslate, well, it’s not evil, it’s just not good, so we wanted to stop using it, however, there’s no real way out of it. Search around and you find this post on their boards DON’T YOU EVEN DARE DO THIS.
It’s just bad programming. You will probably hose your database doing this.