If you work with databases, sooner or later you’ll need to write SQL to “fix” data in a database table (or tables). One key piece of advice is to make sure that the SQL script you come up with is reversible, just in case you need to change things back or make additional changes on the data set that was changed.

Here’s an example of what I mean:

If this was your data conversion script (to flag users as deleted):

update users
set deleted = 1 where id = (
select id from users
where to_be_deleted = 1
and deleted = 0)…