在 SQL Server 中执行 SELECT 语句时使用 INNER JOIN 时，如何只包含一个匹配项？

Question

SELECT distinct
    sr.ServiceDescription,
    os.ServiceID
FROM            
    OrderedServices as os           
INNER JOIN
    Orders as o ON o.OrderID = os.OrderID
INNER JOIN
    ServiceRelation as sr ON os.ServiceID = sr.ServiceID
WHERE 
    o.OrderID = 2802882;

假设这会返回如下内容：

Service Description  | ServiceID
--------------------------------
Basic Cable          | 2
Extended Cable       | 5
ExtendedPlus Cable   | 5
Everything Cable     | 10

但是，如果两个 ServiceIDs 匹配，我只想为 Service Description 返回一个值。看到ServiceID of 5 有两个Service Descriptions？我只想抓住其中一个。如何修改上面的 select 语句来完成这样的事情？

Answer 1

您可以使用MAX()、MIN() 或Row_Number()

SELECT distinct
    max(sr.ServiceDescription),
    os.ServiceID
FROM            
    OrderedServices as os           
INNER JOIN
    Orders as o ON o.OrderID = os.OrderID
INNER JOIN
    ServiceRelation as sr ON os.ServiceID = sr.ServiceID
WHERE 
    o.OrderID = 2802882 group by  os.ServiceID;

或者

SELECT distinct
        MIN(sr.ServiceDescription),
        os.ServiceID
    FROM            
        OrderedServices as os           
    INNER JOIN
        Orders as o ON o.OrderID = os.OrderID
    INNER JOIN
        ServiceRelation as sr ON os.ServiceID = sr.ServiceID
    WHERE 
        o.OrderID = 2802882 group by  os.ServiceID;

或者

 ;with cte as
    (
    SELECT distinct
                sr.ServiceDescription,
                os.ServiceID,
               Row_Number() Over(partition by os.ServiceID order by  os.ServiceID)           
 as rn
            FROM            
                OrderedServices as os           
            INNER JOIN
                Orders as o ON o.OrderID = os.OrderID
            INNER JOIN
                ServiceRelation as sr ON os.ServiceID = sr.ServiceID
            WHERE 
                o.OrderID = 2802882
    )
    select * from cte where rn=1