Delete Duplicate Records, Keep the Original

Three people signed up twice under slightly different names — Daniel Martinez and Dan Martinez, Emma Wilson and Emma W., John Smith and John Smith Jr — but each pair shares one email address.

Delete the duplicates, keeping the lowest customer_id for each email (the original signup). Then show customer_id, name, email for everyone left, ordered by customer_id.

The pattern is worth memorising, because deleting duplicates is one of the most-searched SQL tasks there is: find the id you want to KEEP per group with MIN(customer_id) ... GROUP BY email, then delete every row whose id is NOT IN that set. You are not deleting duplicates directly — you are keeping the survivors and deleting everything else.

Note what makes these hard in real life: the names differ. Deduplicating on name would find nothing. Picking the right key is the actual skill.