SQL：加入两个表，然后使用sum（）和count（）

Question

如果我有两张桌子：

学

school_id | class_id | school_location
-------------------------------------------
400             50               Arizona

人员：

staff_id | forename | school_id | wage
------------------------------------------
1           Peter          400         5000

我怎样才能得出在学校工作的员工人数和每所学校的工资。

例如：

school_id | numberofstaff | salary
---------------------------------------
400 |           1         | 5000

我知道我应该加入这样的表：

SELECT school_id 
FROM school 
INNER JOIN staff
ON school.school_id = staff.school_id

但是我不确定如何进行查询的剩余部分。

SELECT numberofstaff COUNT(numberofstaff) from staff
GROUP BY school_id
Union all 
select 'SUM' numberofstaff, COUNT(numberofstaff)
FROM staff

这应该给我的学校ID以及每个school_id的工作人员数量。

Answer 1

一旦你加入了两张桌子，你就可以统计每个学校的工作人员并计算所有工资：

SELECT sc.school_id, 
   COUNT(*) as numberofstaff,
   SUM(st.wage) as salary
FROM school sc
INNER JOIN staff st
ON sc.school_id = st.school_id
GROUP BY sc.school_id

Answer 2

试试这种方式

SELECT COUNT(staff.numberofstaff) as numberofstaff , school.school_id as school_id FROM school 
INNER JOIN staff
ON school.school_id = staff.school_id
GROUP BY school.school_id;