[SQL] left join, inner join, group by, order by, count

Left join: 왼쪽에 붙이는 것. 해당되는 값이 없는 경우에는 Null로 표기됨 Inner join: 교집합이 나온다. 같이 공유하는 리스트를 중심으로 붙여짐. select * from enrolleds e inner join users u on e.user_id = u.user_id Immerge by course_id, count couse_id and title, and show the head of them. select c1.course_id, c2.title, count(*) as cnt from checkins c1 inner join courses c2 on c1.course_id = c2.course_id group by c1.course_id select pu.user_id , u..

2023.08.16