子查询问题

Question

部（DepartID，DepName）

员工（名称，DepartID）

我需要的是员工与DepName系计数。

Answer 1

如果您正在使用SQL Server版本或以上，这里是另一个可能的方式让员工按部门计数。 。

SELECT  DPT.DepName 
     , EMP.EmpCount 
FROM  dbo.Department DPT 
CROSS APPLY (
       SELECT COUNT(EMP.DepartId) AS EmpCount 
       FROM dbo.Employees EMP 
       WHERE EMP.DepartId = DPT.DepartId 
      ) EMP   
ORDER BY DPT.DepName 

希望有所帮助。

样品测试查询输出：

Answer 2

我会使用outer join而不是子查询。

SELECT d.DepName, COUNT(e.Name) 
FROM Department d 
LEFT JOIN Employees e ON e.DepartID = d.DepartID 
GROUP BY d.DepartID, d.DepName 

Answer 3

SELECT d.DepName, COUNT(e.Name) 
    FROM Department d 
     LEFT JOIN Employees e 
      ON d.DepartID = e.DepartID 
    GROUP BY d.DepName 

Answer 4

无需子查询。

SELECT dep.DepName, COUNT(emp.Name) 
FROM DepName dep 
    LEFT OUTER JOIN Employees emp ON dep.DepartID = emp.DepartID 
GROUP BY dep.DepName 

Answer 5

SELECT COUNT(DISTINCT Name) FROM 
Department AS d, Employees AS e 
WHERE d.DepartID=e.DepartID AND d.DepName = '$thename' 

Answer 6

，并避免使用group by，节省您的queryplan排序操作：

SELECT 
    Department.DepName, 
    (SELECT COUNT(*) 
    FROM Employees 
    WHERE Employees.DepartID = Department.DepartID) 
FROM 
    Department