You could always do a LEFT JOIN and just specify a filter based on your @innerjoin parameter

[font="Courier New"]select *

from tableA ta

left join tableB tb on tb.idA=ta.idA

left join tableC tc on tc.idB=tb.idB

where

ta.param1 = @param1

and (@innerjoin = false or ta.param2 = @param2)

AND (@innerjoin = false or tb.idA is not null)

AND (@innerjoin = false or tc.idB is not null)[/font]