Question

我试图在MS SQL Server中编写一个propper SQL查询。首先，我有以下表格：城镇，员工，地址。几乎每个员工都有Manager，ManagerID也是Employees的外键。（自我关系）我的目标是显示每个城镇的经理人数。到目前为止我有这个代码：

SELECT t.Name, COUNT(*) AS [Managers from each town] 
FROM Towns t
JOIN Addresses a
ON t.TownID = a.TownID
JOIN Employees e
ON a.AddressID = e.AddressID
GROUP BY t.Name
ORDER BY [Managers from each town] DESC

此查询返回每个城镇的员工人数，而不是经理人。如果我尝试下面的第二个查询，我会得到一些完全错误：

SELECT t.Name, COUNT(*) AS [Managers from each town] 
FROM Towns t
JOIN Addresses a
ON t.TownID = a.TownID
JOIN Employees e
ON a.AddressID = e.AddressID
JOIN Employees m
ON e.ManagerID = m.ManagerID

GROUP BY t.Name
ORDER BY [Managers from each town] DESC

以下是＆＃39;员工＆＃39;的结构。表：

EmployeeID，FirstName，LastName，MiddleName，JobTitle，DepartamentID，ManagerID，HireDate，Salary，AddressID

正确的查询必须返回此结果集：

Town          | Managers from each town
Issaquah      | 3
Kenmore       | 5
Monroe        | 2
Newport Hills | 1

Answer 1

如果我正确理解您的结构，那么员工是经理的唯一指示是，其ID是否用作其他员工的经理人。您的第一个查询已正确显示计数，然后所需的只是使用

等过滤结果

where EmployeeID in (select ManagerID from Employees)

因此将您的第一个查询转换为：

SELECT t.Name, COUNT(*) AS [Managers from each town] FROM Towns t
JOIN Addresses a
ON t.TownID = a.TownID
JOIN Employees e
ON a.AddressID = e.AddressID
where EmployeeID in (select ManagerID from Employees)
GROUP BY t.Name
ORDER BY [Managers from each town] DESC

Answer 2

我认为原始查询的以下变体应该是居住在每个城镇的经理：

SELECT t.Name, COUNT(DISTINCT e.EmployeeId) AS [Managers from each town]
FROM Towns t JOIN
      Addresses a
      ON t.TownID = a.TownID JOIN
      Employees e
      ON a.AddressID = e.AddressID
WHERE e.EmployeeId IN (SELECT e2.ManagerId FROM Employees e2)
GROUP BY t.Name
ORDER BY [Managers from each town] DESC;

DISTINCT可能没有必要，但如果不更好地理解数据结构，很难说。

Answer 3

尝试：

System.String[]

Answer 4

您可以尝试以下查询来获取所需的输出...

select t.TownName, COUNT(*) as No from Town t
Inner Join Address a on a.TownID = t.TownID
inner join Employee e on e.ManagerID = a.EmployeeID
Group By t.TownName

在SQL查询中使用GROUP BY的麻烦

4 个答案: