Database

LeetCode 196 note: it must be a DELETE, not a SELECT; keep only the smallest id per email and delete the rest

LeetCode 183 note: LEFT JOIN Orders, and customers with no matching order (o.id IS NULL) never ordered; NOT IN also works

LeetCode 182 note: group by email and keep groups with HAVING COUNT(*) > 1

LeetCode 181 note: self-join Employee on managerId = id to pair employees with managers, then compare salaries

LeetCode 175 note: people without an address must still appear, so LEFT JOIN, leaving city and state NULL when unmatched