mysql - 同一基础表的两个 GROUP BY 子查询的列总和

Question

更新我确定了一种更加简洁且计算上最优的方式来表达这一陈述。没有子查询没有多重连接。:

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)

<小时/>

使用 MySQL Employees Sample Database ，我通过子查询查询同一个员工表两次，并交叉连接结果以获得下面的数据透视表。我希望创建最后一列，该列是 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'

我尝试再次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

表达此语句的一种更加简洁且计算上最佳的方式。没有子查询，没有多重联接，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)