Question

I am trying to write a SQL query where I am joining two tables and retrieving a few columns. Once this is done, based on a two fields (source table enum, id in the corresponding source table), I need to retrieve a value matching the id in the source table.

The issue is that I cannot join all the source tables, and want to do something like this:

Select 
  X.Col1,
  X.Col2,
  Y.Col1,
  Y.Col2,
  CASE
    WHEN Y.TableID = 4 THEN Select t4.Col1 FROM TableFour t4 WHERE t4.id = Y.FileID
    WHEN Y.TableID = 5 THEN Select t5.Col4 FROM TableFive t5 WHERE t5.id = Y.FileID
  END
FROM Table X INNER JOIN Table Y ON X.ID = Y.XID

I can guarantee the value being retrieved from all the source tables will be the same (i.e. nvarchar).

Nesting SQL queries inside the CASE statement doesn’t seem to work, I am throwing it you guys. Any ideas with this problem?

Hope I explained the question adequately. If you are unsure, make a comment so I can clarify it.

Cheers in advance!

Answer 1

I don’t understand what do you mean by “cannot join all the source tables”, can you elaborate?
I would use outer joins:

Select 
  X.Col1,
  X.Col2,
  Y.Col1,
  Y.Col2,
  COALESCE(t4.Col1 ,t5.Col4)
  END
FROM Table X INNER JOIN Table Y ON X.ID = Y.XID
LEFT OUTER JOIN TableFour t4 ON t4.id = Y.FileID AND Y.TableID = 4
LEFT OUTER JOIN TableFive t5 ON t5.id = Y.FileID AND Y.TableID = 5