Question

Is there a way in SQL Server 2008 do to something like below?

Can the outer applied(joined) table to be specified based on a condition?

declare @bGetExtendedInfo bit
set @bGetExtendedInfo = 1

declare @param nvarchar(24)
set @param = 'CO-02-BBB'

select t1.*, t2.ID
from t1
outer apply (
              case when @bGetExtendedInfo= 0 
                  then (select 0) as ID  /* dummy value */
                  /*really expensive query trying to avoid when extended info is not needed*/
                  else (select top 1 ID from tbl1 where tbl1.code = @param)
              end
             ) t2

Answer 1

You can readily do this with just a join:

SELECT t1.*, t2.*
FROM   t1 cross join
       (SELECT *
        FROM (SELECT top 1 tb10.*, 0 as bCcond src FROM tb10 
              UNION ALL
              SELECT top 1 tb11.*, 1 as bCcond src FROM tb11
             ) t
        WHERE @bCond = bCcond
       ) t2

Doing the top before the union all should also help the optimizer produce a better query plan, in the event that the tables are really complex joins.

Also, doing top without an order by is generally frowned upon. It can return different rows with different invocations, but the rows are not guaranteed to be random.