Question

我们将不得不基于表1列项目类型联接4个表。

表A包含id，itemtype，itemid

表B包含ID，全名

表c包含ID，全名

表D包含ID，全名

表A.itemid位于ID的（表B或表C或表D）中。

我想将表A与restg表连接起来以获得全名。基于TableA列的itemtype值联接另一个表

     select
     tblA.itemid,y.fullname from tableA as tblA 
      inner join (
        CASE WHEN tblA.itemtype = 1 THEN 
          select
            tblB.itemid as id,tblB.fullname  as fullname
          from
            tableB    as tblB 
          where
            tblB.id = tblA.itemid
        WHEN tblA.itemtype = 2 THEN  
          select
            tblC.itemid as id,tblC.fullname  as fullname
          from
            tableC  as tblC  
          where
            tblC.id = tblA.itemid
            WHEN tblA.itemtype = 3 THEN 
              select
            tblD.itemid as id ,tblD.fullname as fullname
          from
            tableD  as tblD  
          where
            tblD.id = tblA.itemid

          END
      ) as bcd on bcd.id = tblA.itemid

Answer 1

您可以left join表B，C和D到表A，并在{{1}中包含列itemtype的条件}子句：

ON

请参见demo。
或使用UNION ALL：

select
  A.itemid,
  coalesce(B.fullname, C.fullname, D.fullname) fullname
from A 
left join B on B.id = A.itemid AND A.itemtype = 1
left join C on C.id = A.itemid AND A.itemtype = 2
left join D on D.id = A.itemid AND A.itemtype = 3

请参见demo。

Answer 2

如果关联表中的列不多：

SELECT
   a.itemid,
   COALESCE(b.fullname, c.fullname, d.fullname, ' - Missing name - '))
FROM
   tblA a 
   LEFT JOIN tblB b ON a.itemtype = 1 AND a.itemid = a.id
   LEFT JOIN tblC c ON a.itemtype = 2 AND a.itemid = b.id
   LEFT JOIN tblD d ON a.itemtype = 3 AND a.itemid = c.id

带大小写的表连接

2 个答案: