mysql - 统计主管的员 worker 数

Question

SELECT 
e.lname AS employee_name
, s.lname AS supervisor_name
, e.superssn AS supervisor_ssn
FROM employee e INNER JOIN employee s
WHERE e.superssn=s.ssn

其实我有两个问题。第一个是，上面的语句很好用。然而现在它会显示一群员工和他们的主管。组织得不好。顺便说一句，监事一共有三名。有没有办法显示主管 A 与员工 A 以及主管 B 与员工 B 等等？

第二个问题是我也尝试只计算每个主管的员工数量，而不是用 COUNT() 显示他们的名字，我尝试了几种不同的方法但没有成功。

Answer 1

一些事情

1) 最好将连接谓词与连接一起保留。所以改变:

FROM employee e INNER JOIN employee s WHERE e.superssn = s.ssn

致:

FROM employee e INNER JOIN employee s ON e.superssn = s.ssn

2) 要由主管将它们放在一起，只需使用 order by:

SELECT 
  e.lname AS employee_name
  , s.lname AS supervisor_name
  , e.superssn AS supervisor_ssn
FROM employee e INNER JOIN employee s ON e.superssn = s.ssn
ORDER BY s.lname

3) 要计算每个主管的员工数量，请使用 group by 和 COUNT 聚合函数

SELECT 
  s.lname AS supervisor_name,
  COUNT(*) AS employee_count
FROM employee e INNER JOIN employee s ON e.superssn = s.ssn
GROUP BY s.lname
ORDER BY s.lname