예시
kakaopay로 결재한 유저 정보
select u.user_id , u.email, u.name from users u
inner join orders o on u.user_id = o.user_id
where payment_method = 'kakaopay'
위 쿼리문을 subquery로 작성해 보면 아래와 같다.
select u.user_id , u.email, u.name from users u
where u.user_id in (
select o.user_id from orders o
where o.payment_method = 'kakaopay'
)