在SQL中显示员工姓名（Boss）和员工（下属）的数量

Question

我有一个表emp有foll数据：

EmpID    EmpName   MgrID 

100     King        NULL
101     Smith       100
102     Shine       100
103     Racy        102

现在我要显示员工姓名（Boss）和员工（下属）的数量

BOSS       SUBORDINATES

---

BLAKE                 5

CLARK                 1

FORD                  1

JONES                 2

KING                  3

SCOTT                 1

请指导如何在SQL Server 2008中查询此表。

尝试查询：

select e.first_name as ename,m.first_name as mname from employees e,employees m where e.manager_id=m.employee_id

Answer 1

• 首先在EmpID=MgrID
上自行加入
• 按MgrID和EmpName
分组
• 选择EmpName和count(*)

将其翻译为SQL是机械的：

SELECT b.EmpName, COUNT(*)
FROM Employee e
JOIN Employee b ON b.EmpID=e.MgrID
GROUP BY b.EmpID, b.EmpName

Answer 2

CREATE TABLE test (
   EmpID INT,
   EmpName VARCHAR(100),
   MgrID INT)

INSERT INTO test VALUES (100, 'King', NULL),
                        (101, 'Smith', 100),
                        (102, 'Shine', 100),
                        (103, 'Racy', 102)

SELECT t1.EmpName AS Boss, 
       COUNT(*) AS Subordinates
FROM test AS t1 INNER JOIN test AS t2 ON t1.EmpID = t2.MgrID
GROUP BY t1.EmpName