Question

我编写了一个查询，该查询可以找到特定班级性别的学生人数，但是我无法找到一种方法可以对两种性别都做相同的查询。我尝试两次（针对每个性别）编写查询两次，然后使用UNION将它们组合在一起，但也无法做到这一点。

查询＃1

SELECT MAX(Class.Class_ID) AS Class_ID,  COUNT(Enrolled_In.Reg_No) AS Male_Students
FROM Enrolled_In
INNER JOIN Class ON Enrolled_In.Class_ID = Class.Class_ID
INNER JOIN Student ON Enrolled_In.Reg_No = Student.Reg_No
WHERE Class.Class_ID = 'E-100' AND Student.Gender = 'M';

结果：

CLASS_ID MALE_STUDENTS
-------- -------------
E-100               2

查询＃2

SELECT MAX(Class.Class_ID) AS Class_ID,  COUNT(Enrolled_In.Reg_No) AS Female_Students
FROM Enrolled_In
INNER JOIN Class ON Enrolled_In.Class_ID = Class.Class_ID
INNER JOIN Student ON Enrolled_In.Reg_No = Student.Reg_No
WHERE Class.Class_ID = 'E-100' AND Student.Gender = 'F';

结果：

CLASS_ID FEMALE_STUDENTS 
-------- ---------------
E-100                 1

我希望输出是这样的，最好使用单个查询：

CLASS_ID MALE_STUDENTS FEMALE_STUDENTS
-------- ------------- --------------- 
E-100               2                1

谢谢。

Answer 1

SELECT SUM(case when s.Gender = 'M' then 1 else 0 end) AS Male_Students,
       SUM(case when s.Gender = 'F' then 1 else 0 end) AS Female_Students
FROM Enrolled_In e
JOIN Student s ON e.Reg_No = s.Reg_No
WHERE e.Class_ID = 'E-100'

Answer 2

尝试以下

select
  c.class_id, 
  sum(case when s.gender = 'm' then 1 else 0 end) as male_students,
  sum(case when s.gender = 'f' then 1 else 0 end) as female_students
from enrolled_in ei
join class c
on ei.class_id = c.class_id
join student s
on ei.reg_no = s.reg_no
where c.class_id = 'e-100'
group by
  c.class_id

如何将两个SQL查询与不同的条件组合在一起？

2 个答案: