Question

我有一个表comments，其中包含一个字段student_id（students表的外键。我有另一张桌子students

我想做的是运行查询，显示所有未发表评论的学生。我有的SQL，只显示已发表评论的学生

SELECT studentID, email, first_name, last_name FROM "students" JOIN comments ON students.id = comments.student_id

如何“反向”此SQL以显示尚未评论的学生？

Answer 1

一种方法使用not exists：

select s.*
from students s
where not exists (select 1
                  from comments c
                  where s.id = c.student_id
                 );

Answer 2

你可以这样做：

SELECT studentID, email, first_name, last_name
FROM students
LEFT JOIN comments ON students.id = comments.student_id
WHERE comments.student_id IS NULL

Answer 3

select s.studentID, s.email, s.first_name, s.last_name from students s where s.id not in (select student_id from comments);

如何在SQL中选择“例外”？

3 个答案: