使用 ID T-SQL 多重连接到单列

Question

我有以下问题。我正在尝试连接三个表：

表A：Record_id、Reference_id、参数

表 B：Doc_id，名称 B

表 C：Doc_id，名称 C

根据 TableA.Param 中的值，我需要 select 通过加入 Reference_id

来自表 B 或 C 的名称

我尝试过使用 "case" 约束，但没有成功 :)

select  a.Record_id, a.Reference_id
       , case when Param = 'B' then b.NameB
              when Param = 'C' then c.NameC
from TableA as a inner join
     TableB as b on a.Reference_id = b.Doc_id inner join
     TableC as c on a.Reference_id = c.Doc_id

有什么想法吗？

Answer 1

您可以 left join 满足该条件并使用 coalesce()

获取第一个非空名称

select  a.Record_id, a.Reference_id,
        coalesce(b.NameB, c.NameC) as name
from TableA as a 
left join TableB as b on a.Reference_id = b.Doc_id 
                     and Param = 'B'
                     and b.STAT = 'A'
left join TableC as c on a.Reference_id = c.Doc_id 
                     and Param = 'C'
                     and c.STAT = 'A'

顺便说一句，您的查询已结束（错过了 end）。这适用于 left joins

select  a.Record_id, a.Reference_id,
        case when Param = 'B' then b.NameB
             when Param = 'C' then c.NameC
        end as name
from TableA as a 
left join TableB as b on a.Reference_id = b.Doc_id and b.STAT = 'A'
left join TableC as c on a.Reference_id = c.Doc_id and c.STAT = 'A'

Answer 2

试试这个：

SELECT a.Record_id, a.Reference_id, b.NameB AS Name
FROM TableA AS a
INNER JOIN TableB AS b ON a.Reference_id = b.Doc_id 
WHERE a.Param = 'B'

UNION

SELECT a.Record_id, a.Reference_id, c.NameC AS Name
FROM TableA AS a
INNER JOIN TableC as c ON a.Reference_id = c.Doc_id 
WHERE c.Param = 'C'

Answer 3

SELECT  a.Record_id, a.Reference_id,V.Name
FROM TableA AS a 
inner join
(
SELECT Doc_id id, NameB Name,'B' Type from TableB
UNION ALL
SELECT Doc_id ID, NameC name,'C' Type  From TableC
) AS V On V.ID=Reference_id AND Type=Param

使用 ID T-SQL 多重连接到单列

Multiple Join to single column using ID T-SQL

sql

join

left-join