It takes all purchases records and joins a corresponding user to each record, but only if it has a specific id. Otherwise it’s still every purchases record. But

It takes all purchases records and joins a corresponding user to each record, but only if it has a specific id. Otherwise it’s still every purchases record. But only one user gets joined. Join condition is not a filter, it is a join condition. Left join will always yield all left-table rows. Right join will always yield all right-table rows. “Where” works on a final result of select+joins (but before group by), so if you want to filter by user id, the condition should go in where.