如何在复杂条件下从表中选择值？

Question

我有一个3列的表：course_name, year, semester。

现在我想找出在每学期(2001年和2002年)有课程的科目(S1和S2)。

我试着写了一个小时的CASE WHEN和GROUP BY HAVING，但是没有得到正确的结果。

table_subjects

course_name| year| semester
Programming   2001   S1
Programming   2001   S2
Programming   2002   S1
Programming   2002   S2
Law           2001   S1
Law           2001   S2
Law           2002   S2
Science       2001   S1
Science       2001   S2
Management    2002   S2
AI            2001   S1
Database      2001   S1
Database      2001   S2
Database      2002   S1
Database      2002   S2

预期结果：

|course_name|
 Programming
 Database

Answer 1

你可以在下面试试-

演示

select course_name
from t1 a
where year in (2001,2002) and exists (select 1 from t1 b where a.course_name=b.course_name
and a.year=b.year and semester in ('S1','S2') having count(distinct semester)=2)
group by course_name
having count(distinct year)=2 

产出：

course_name
Database
Programming

Answer 2

SELECT T.course_name
FROM
(
    SELECT course_name
    FROM table_subjects
    GROUP BY course_name,
             year
    HAVING COUNT(1) = 2
) T
GROUP BY T.course_name
HAVING COUNT(1) = 2;