Question

我有一个场景，我想在一个连接上放一个条件，即如果A = B然后加入1 set，另一个加入另一个。两种情况下的表格相同，只是条件不同。我尝试过使用大小写，但是我遇到了语法错误。

SELECT * 
FROM [Table1] CUP
LEFT OUTER JOIN Table2 CP ON CUP.StateCode = CP.StateCode 
                          AND (CUP.ClientId = CP.ClientId OR CP.ClientId = 0)

我尝试了什么

SELECT * 
FROM [Table1] CUP
LEFT OUTER JOIN Table2 CP
          ON CASE
                WHEN CUP.ClientId =  CP.ClientId 
                   THEN CUP.StateCode = CP.StateCode AND CUP.ClientId =  CP.ClientId
                   ELSE CUP.StateCode = CP.StateCode AND (CUP.ClientId <>  CP.ClientId or CP.ClientId = 0)

@juan

 ON 
   CUP.StateCode = CP.StateCode AND ((CUP.ClientId =  CP.ClientId or CP.ClientId <> 0) OR (CUP.ClientId <> CP.ClientId or CP.ClientId = 0))

Answer 1

您必须使用coalesce作为SELECT条款
请注意CP1.ClientId is null删除了Gordon解决方案有关NULLS的限制

select  ...

from                    table1  CUP

        left outer join Table2  CP1

        on              CP1.StateCode = CUP.StateCode  
                    and CP1.ClientId  = CUP.ClientId 

        left outer join Table2  CP2

        on              CP1.ClientId is null

                    and CP2.StateCode = CUP.StateCode 
                    and CP2.ClientId  = 0

Answer 2

我写了这个：

SELECT * 
  FROM   TABLE1 CUP
  INNER JOIN TABLE2 CP ON CUP.StateCode = CP.StateCode 
   AND CUP.ClientId =  CP.ClientId   
  UNION ALL
  SELECT * 
  FROM (SELECT * FROM TABLE1 WHERE NOT EXISTS (SELECT 1 FROM TABLE2 WHERE TABLE2.CLIENTID = TABLE1.CLIENTID) ) CUP 
  INNER JOIN TABLE2 CP  ON    CUP.StateCode = CP.StateCode AND CP.CLIENTID=0

我用这些数据试了一下：

CREATE TABLE TABLE1  (STATECODE INT, CLIENTID INT);
CREATE TABLE TABLE2  (STATECODE INT, CLIENTID INT);
INSERT INTO TABLE1 VALUES (1,100);
INSERT INTO TABLE1 VALUES (1,200);
INSERT INTO TABLE1 VALUES (4,300);
INSERT INTO TABLE1 VALUES (5,400);
INSERT INTO TABLE1 VALUES (4,500);

INSERT INTO TABLE2 VALUES (1,100);
INSERT INTO TABLE2 VALUES (1,200);
INSERT INTO TABLE2 VALUES (4,0);
INSERT INTO TABLE2 VALUES (5,0);

输出：

STATECODE   CLIENTID    STATECODE   CLIENTID
----------- ----------- ----------- -----------
1           100         1           100
1           200         1           200
4           300         4           0
5           400         5           0
4           500         4           0

Answer 3

我认为你想要两个left join，一个是第一个值，第二个是默认值：

SELECT CUP.*, COALESCE(CP.col1, CPdef.col1) as col1, . . .
FROM [Table1] CUP LEFT OUTER JOIN
     Table2 CP
     ON CUP.StateCode = CP.StateCode AND CUP.ClientId = CP.ClientId LEFT OUTER JOIN
     Table2 CdPef
     ON CUP.StateCode = CPdef .StateCodeCP AND CPdef.ClientId = 0;

请注意，COALESCE()假定您关注的列不是NULL天。

如果您只需要一次匹配，则另一种方法是outer apply：

SELECT CUP.*, cp.*
FROM [Table1] CUP OUTER APPLY
     (SELECT top 1 cp.*
      FROM Table2 Cp
      WHERE CUP.StateCode = CP.StateCode AND
            (CUP.ClientId = CP.ClientId OR CP.ClientId = 0)
      ORDER BY CP.ClientId DESC  -- assumes `CP.ClientId` is not null
     ) cp;

实际上，您可以使用TOP (1) WITH TIES来获得多个匹配项。所以这不仅限于一个匹配的行。

Answer 4

当一个案例全部说完并完成时，它以一个值结束。

case when SomeColumn = 4 then 1 end

如您所见，返回值为VALUE

您可以使用它来返回连接中的列，但实际上您要返回列

中的值

select aa.a
from aa
inner join bb
on aa.a = case when bb.a is null then 4 else bb.a end

您不能使用案例来返回表达式

case when aa.a = 4 then (aa.a = bb.a) end -- this is gonna fail

在这些情况下使用OR：

SELECT * 
FROM [Table1] CUP
LEFT OUTER JOIN Table2 CP
  ON (CUP.ClientId =  CP.ClientId 
      and CUP.StateCode = CP.StateCode )
  or (CUP.StateCode = CP.StateCode 
      AND (CUP.ClientId <>  CP.ClientId 
      or CP.ClientId = 0))

SQL Server：左外连接上的条件（IF-ELSE）

4 个答案: