同一基础表的两个GROUP BY子查询中的列的总和

Question

UPDATE我确定了一种更简洁、计算优化的方式来表达这句话。没有子查询，没有多个联接。：

SELECT
month(hire_date) as month, 
sum(case when gender ="m" then 1 end) as male,
sum(case when gender ="f" then 1 end) as female,
count(*) as total 
FROM employees
GROUP BY MONTH(hire_date)

---

使用MySQL员工示例数据库，我通过子查询和交叉连接结果两次查询同一个员工表，以获得下面的透视表。我希望创建一个最后的列，它将是mcount和fcount的总和

Month,Gender,mcount,gender,fcount
'1','M','14310','F','9549'
'2','M','14659','F','9789'
'3','M','16200','F','10717'
'4','M','15363','F','10190'
'5','M','15545','F','10398'
'6','M','15000','F','10003'
'7','M','15260','F','10345'
'8','M','15250','F','10146'
'9','M','14632','F','9684'

我再次尝试使用子查询来创建此列，但很明显，我的两个GROUP BY子查询是构建此数据透视表的错误方法。

SELECT mbm.month, mbm.gender, mbm.count as mcount, fbm.gender, fbm.count as fcount
FROM
(SELECT MONTH(hire_date) as Month, gender, COUNT(*) as count
FROM employees
WHERE gender = "M"
GROUP BY gender,MONTH(hire_date)
) as mbm
CROSS JOIN (SELECT MONTH(hire_date) as Month, gender, COUNT(*) as count
FROM employees
WHERE gender = "F"
GROUP BY gender,MONTH(hire_date)
) AS fbm on mbm.Month = fbm.Month

Answer 1

一种更简洁、计算更优化的表达此语句的方式。没有子查询，没有多个联接，并且易于阅读：

SELECT
month(hire_date) as month, 
sum(case when gender ="m" then 1 end) as male,
sum(case when gender ="f" then 1 end) as female,
count(*) as total 
FROM employees
GROUP BY MONTH(hire_date)