使用ID T-SQL多次连接到单列

Question

I have following issue. I'm trying to join three tables:

TableA: Record_id, Reference_id, Param

TableB: Doc_id, NameB

TableC: Doc_id, NameC

depending on value in TableA.Param, I need to select Name from tables B or C by joining on Reference_id

I've tried to use "case" constrain, but didn't worked out :)

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

Any ideas?

Answer 1

You could left join with that condition and take the first non-null name with coalesce()

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'

BTW your query was close (missed the end ). This would have worked with left join s

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

Try this:

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