Sql 在对联接表使用COUNT（）时是否包含其他字段？

我将学习SQL，并在其中一个练习中遇到麻烦。“获取每位员工负责的客户数量”（我希望翻译可以）。我发现了以下几点：

SELECT emp.EmployeeId, COUNT(cus.SupportRepId) AS Customers
FROM Employee AS emp JOIN Customer AS cus 
ON cus.SupportRepId = emp.EmployeeId
GROUP BY emp.EmployeeId

到目前为止，结果是正确的：

EmployeeId      Customers
-------------------------
3               21
4               20
...             ...

现在，我认为在结果中添加员工的数据（例如“employee”表中的JobTitle）也不错，但这似乎不起作用：

SELECT emp.EmployeeId, emp.JobTitle, COUNT(cus.SupportRepId) AS Customers
FROM Employee AS emp JOIN Customer AS cus 
ON cus.SupportRepId = emp.EmployeeId
GROUP BY emp.EmployeeId

我不明白为什么。为了获得预期结果，我应该做什么：

EmployeeId      JobTitle              Customers
------------------------------------------------
3               Key Account Manager   21
4               Business Area Manager 20
...             ...

---

我希望你能帮忙。在此之前非常感谢。

您可以通过以下方式将其放入

组：

SELECT e.EmployeeId, e.JobTitle, COUNT(c.SupportRepId) AS Customers
FROM Employee e JOIN
     Customer c 
     ON c.SupportRepId = e.EmployeeId
GROUP BY e.EmployeeId, e.JobTitle;

或者，您可以在加入之前使用相关子查询或聚合：

SELECT e.*, c.Customers
FROM Employee e JOIN
     (SELECT c.SupportRepId, COUNT(c.SupportRepId) AS Customers
      FROM Customer c 
      GROUP BY c.SupportRepId
     ) c
     ON c.SupportRepId = e.EmployeeId;