mysql 组函数count的具体用法

Question

SELECT d.department,
COUNT(CASE e.sex WHEN e.sex='f' AND p.position !='主管' THEN p.id END) fq,
COUNT(CASE e.sex WHEN e.sex='f' AND p.position ='主管' THEN p.id END) fz,
COUNT(CASE e.sex WHEN e.sex='m' AND p.position !='主管' THEN p.id END) mq,
COUNT(CASE e.sex WHEN e.sex='m' AND p.position ='主管' THEN p.id END) mz,
COUNT(e.id) xj
FROM department d LEFT JOIN employ e ON d.id=e.deptid
LEFT JOIN position p ON e.positionid=p.id
GROUP BY d.id;

出现后面两个count统计不出来是什么原因？
为什么下面个可以查出来？
SELECT d.department,
COUNT(CASE WHEN e.sex='f' AND p.position !='主管' THEN p.id END) fq,
COUNT(CASE WHEN e.sex='f' AND p.position ='主管' THEN p.id END) fz,
COUNT(CASE WHEN e.sex='m' THEN CASE WHEN p.position !='主管' THEN p.id END END) mq
COUNT(CASE WHEN e.sex='m' THEN CASE WHEN p.position ='主管' THEN p.id END END) mz,
COUNT(e.id) xj
FROM department d LEFT JOIN employ e ON d.id=e.deptid
LEFT JOIN position p ON e.positionid=p.id
GROUP BY d.id

Answer 1

SELECT CASE 1 WHEN 1=1 THEN 'a' else 'b' END;
SELECT CASE 2 WHEN 1=1 THEN 'a' else 'b' END;

是你的理解错误！上面的两行脚本 我想你以为都等于a吧？
其实思路就跟你的一样。也可以这样写

SELECT CASE 1=1 WHEN 1=1 THEN 'a' else 'b' END;
SELECT CASE 2=2 WHEN 1=1 THEN 'a' else 'b' END;

而结果却都是'a' 道理一样