Yeah, wish I'd learned that the easy way. Fresh into one of my first jobs I was working with a vendor's custom interface to merge/purge duplicate records. It didn't have a good method of record matching on inserts from the customer web interface so a large % of records had duplicates.

Anyway, I selected what I though was a "merge all duplicates" option without previewing results. What I had actually done was "merge all selected". So, the system proceeded to merge a very large % of the database... Into One. Single. Record.

Luckily the vendor kept very good backups, and so I kept my job. Because I also luckily had a very good boss and I had already demonstrated my value in other ways, he just asked me "Well, are you going to make that mistake again?". I wisely said no, and he just smiled and said "Then I think we're done here."

I have been particularly fortunate throughout my career to have very good managers. As much as managers get a lot of flack here on HN, done well they are empowering, not a hindrance, and I attribute a lot of success in my career to them.

At a previous job the DB admin mandated that everyone had to write queries that would create a temporary table containing a copy of all the rows that needed to be deleted. This data would be inspected to make sure that it was truly the correct data. Then the data would be deleted from the actual table by doing a delete that joined against the copied table. If for some reason it needed to be restored, the data could be restored from the copy.