[maweaver]

I've had the "joy" of doing data corrections on production databases before. Things I've learned:

- ALWAYS do the work inside of a transaction, as he mentioned. Rollback is your friend. For final run I always do a rollback and "clean" run just to make sure nothing extra slips in.

- Start by crafting your delete/updates as SELECTS. Make sure it targets the data you want then save the SELECT to run afterwards.

- Export any data you plan to modify off to a spreadsheet and save it somewhere. This is another chance to check it's what you intended to change, it's a record of what was changed, and it can be used to revert the data if needed.

- As the author mentioned, get another pair of eyes if possible. Besides making it less likely for things to go wrong, people are more understanding.

- And it probably goes without saying, but don't just fix the data and move on. Do everything possible to track down the bug that caused the issue in the first place and fix it. This is another place where having previous data available in a spreadsheet (or even a complete database backup) can help with data mining after the fact.