如何将两个不同where条件的SQL查询组合起来？

Question

我编写了一个查询，用于查找特定班级中某一性别的学生人数，但我无法找到一种方法来编写可以对两种性别执行相同操作的查询。我尝试编写两次查询（针对每个性别），然后使用UNION将它们组合起来，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

我希望输出如下，最好使用一个查询:

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

谢谢你。

Answer 1

尝试以下方法

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

Answer 2

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'