SELECT id + 1 available_id FROM customers t WHERE NOT EXISTS (SELECT * FROM customers WHERE id = t.id + 1 ) ORDER BY id https://stackoverflow.com/questions/1312101/how-do-i-find-a-gap-in-running-counter-with-sql
SELECT id + 1 available_id FROM customers t WHERE NOT EXISTS (SELECT * FROM customers WHERE id = t.id + 1 ) ORDER BY id https://stackoverflow.com/questions/1312101/how-do-i-find-a-gap-in-running-counter-with-sql