if else within CTE?

Question

I want to execute select statement within CTE based on a codition. something like below

;with CTE_AorB
(
  if(condition)
    select * from table_A
   else
           


        

Answer 1

If you are using a parameter, then you only need one statement.

@ID (Some parameter)

;with CTE
(
    select * from table_A WHERE id = @ID
    union all
    select * from table_B WHERE (id = @ID and condition)
)