Как найти дублирующиеся записи в SQL?

Поиск дублей — частая задача QA. По одному полю: SELECT email, COUNT(*) FROM users GROUP BY email HAVING COUNT(*) > 1. По нескольким полям: SELECT first_name, last_name, COUNT(*) FROM customers GROUP BY first_name, last_name HAVING COUNT(*) > 1. Получить сами дубли: SELECT * FROM users WHERE email IN (SELECT email FROM users GROUP BY email HAVING COUNT(*) > 1). Найти дубли с разными ID — ROW_NUMBER: WITH dups AS (SELECT *, ROW_NUMBER() OVER (PARTITION BY email ORDER BY id) rn FROM users) SELECT * FROM dups WHERE rn > 1.