統計主管的員工人數

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 

我其實有兩個問題。第一個是，上面的聲明工作得很好。但是現在，它會顯示一羣員工和他們的主管。組織不良。順便說一下，有三名主管。有什麼辦法可以讓我們說A和A，B和B，B和B？

第二個問題是，我也嘗試只計算每個主管的員工數量，而不是用COUNT（）顯示他們的名字，我嘗試了幾個不同的但不工作的員工。

Answer 1

有幾件事情

1）這是很好的做法，讓您的加入謂詞您加入。所以改變：

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

要：

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

2）通過主管讓他們在一起，只需使用一個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）做員工的每一個的數主管使用group by和COUNT集合函數

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