Clean it

Before you can join or aggregate this warehouse, you have to trust it — and a warehouse this size never arrives clean. riders has null emails, four different spellings of the same city, and duplicate rows from a signup form that got submitted twice. rides has foreign keys pointing at riders that don't exist. None of this is invented for the exercise — it's the ordinary residue of a table that's been imported, re-imported, and patched for a year. Every step below is graded: write the query, run it, and the check tells you whether you found the real thing.