Question

相关表格：
DepartmentPhone：DepartmentPhoneID int，DepartmentID int，PhoneID int
电话：PhoneID int，PhoneType int

PhoneType = 4的手机有6部属于DepartmentID = 2。所以这产生了6条记录：

select *
from DepartmentPhone
  join Phone on Phone.PhoneID = DepartmentPhone.PhoneID and Phone.PhoneType = 4
where DepartmentPhone.DepartmentID = 2

请注意，DepartmentID = 2仅用于说明目的，我的查询将带来所有部门我想要实现的是为每个部门选择第一部电话（类型= 4） - 每个部门只有1行。我认为以下查询可以解决问题，但它会继续检索所有6条记录。我错过了什么？

select x.*
from DepartmentPhone x
where 
  x.DepartmentID = 2
  and x.PhoneID = (select max(y.PhoneID)
                   from departmentphone y 
                     join Phone on y.PhoneID = Phone.PhoneID and Phone.PhoneType = 4
                   where x.DepartmentPhoneID = y.DepartmentPhoneID)

感谢您的帮助!!!

Answer 1

我希望是一个干净的语法。最好的方法是使用ROW_NUMBER：

;WITH DepartmentPhone_CTE AS
(
    SELECT p.*, 
        ROW_NUMBER() OVER
            (PARTITION BY dp.DepartmentID ORDER BY dp.PhoneID) AS RowNum
    FROM DepartmentPhone dp
    INNER JOIN Phone p
        ON p.PhoneID = dp.PhoneID
    WHERE p.PhoneType = 4
)
SELECT dp.*
FROM DepartmentPhone_CTE
WHERE RowNum = 1

Answer 2

我不了解您的架构，但我猜您需要按DepartmentID关联您的分组，而不是DepartmentPhoneID。

select x.*
from DepartmentPhone x
where 
  x.DepartmentID = 2
  and x.PhoneID = (select max(y.PhoneID)
                   from departmentphone y 
                     join Phone on y.PhoneID = Phone.PhoneID and Phone.PhoneType = 4
                   where x.DepartmentID = y.DepartmentID);

这里有几个替代查询，它们应该在不使用相关子查询的情况下得到相同的结果。第一个使用派生表：

select *
from DepartmentPhone x
join (select d.DepartmentID, max(d.PhoneID) as maxPhoneID
      from DpartmentPhone d join Phone p using (PhoneID)
      where p.PhoneType = 4
      group by d.DepartmentID) y
  using (DepartmentID);

第二种方法根本不使用子查询，而是使用自连接：

select d1.*
from DepartmentPhone d1
join Phone p1 on d1.PhoneID = p1.PhoneID and p1.PhoneType = 4
left outer join (DepartmentPhone d2 join Phone p2 
    on d2.PhoneID = p2.PhoneID and p2.PhoneType = 4)
  on d1.DepartmentID = d2.DepartmentID and d1.PhoneID < d2.PhoneID
where d2.DepartmentID is NULL;

SQL - 帮助“按组排名第一”

2 个答案: