sql - 我需要编写一个查询来列出所有员工以及他们拥有的下属人数

Question

此查询还应显示没有下属的员工应列为零下属。我目前可以返回所有下属的员工，但我似乎无法显示下属为零的员工。

这是到目前为止的代码：

SELECT s.empno, s.ename, COUNT(*) as "Num_subordinates"
FROM emp e
JOIN emp s ON s.empno=e.super 
GROUP BY s.empno, s.ename;

Answer 1

您的陈述中唯一缺少的是使用LEFT JOIN而不是INNER JOIN. 您还需要在计入时指定ID而不是，这样如果员工没有下属，您就不会屈服。*LEFT JOIN1COUNT()

SELECT e.empno, e.ename, COUNT(s.empno) as "Num_subordinates"
FROM   emp e
       LEFT JOIN emp s ON s.empno = e.super 
GROUP  BY e.empno, e.ename

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• SQL 连接的可视化表示

Answer 2

更新

假设数据类似于以下

| EMPNO |     ENAME |  SUPER |
------------------------------
|     1 |  Manager1 | (null) |
|     2 | Employee1 |      1 |
|     3 | Employee2 |      1 |
|     4 | Employee3 |      1 |
|     5 |  Manager2 | (null) |
|     6 | Employee5 |      5 |
|     7 | Employee6 | (null) |

带有子查询的版本

SELECT e.empno, 
       e.ename, 
       (SELECT COUNT(*) 
          FROM emp 
         WHERE super = e.empno) "Num_subordinates"
  FROM emp e;

带有JOIN. 您必须使用LEFT JOINsinceINNER JOIN过滤掉必要的行，正如 JW 正确指出的那样，您需要COUNTons.empno而不是*.

SELECT e.empno,
       e.ename,
       COUNT(s.empno) "Num_subordinates"
  FROM emp e LEFT JOIN emp s
    ON s.super = e.empno
 GROUP BY e.empno, e.ename
 ORDER BY e.empno

两个查询的输出

| EMPNO |     ENAME | NUM_SUBORDINATES |
----------------------------------------
|     1 |  Manager1 |                3 |
|     2 | Employee1 |                0 |
|     3 | Employee2 |                0 |
|     4 | Employee3 |                0 |
|     5 |  Manager2 |                1 |
|     6 | Employee5 |                0 |
|     7 | Employee6 |                0 |

SQLFiddle（对于两个查询）

Answer 3

A few solutions for you problem:

1) grouped auto-join:

SELECT s.empno, s.ename, COUNT(*) as "Num_subordinates"
FROM emp e RIGHT JOIN emp s ON s.empno=e.super 
GROUP BY s.empno, s.ename;

2) left join with aggregate subordinate counts:

SELECT s.*, c.num_subordinates as "Num_subordinates"
FROM emp s LEFT JOIN (SELECT super AS empno, COUNT(*) FROM emp GROUP BY super) AS c
  ON c.empno = s.empno;

or using CTE:

WITH c AS (SELECT super AS empno, COUNT(*) FROM emp GROUP BY super)
SELECT s.*, c.num_subordinates as "Num_subordinates"
FROM emp s LEFT JOIN c
  ON c.empno = s.empno;

3) using scalar sub-query:

SELECT s.*, (SELECT COUNT(*) FROM emp WHERE super=s.empno) AS "Num_subordinates"
FROM emp s;