Is there a way to do a join with cases

Question

Is there a way to make a join with cases?

Something like

select * from abba 
   case when a=b then 
      join acdc on abba.id=acdc.id
   when b=c then 
      join bebop on abba.id=bebop.id 
   end

Is there a way to do this?

Best regards Joe

Answer 1

The usual way to do this is to LEFT JOIN both tables and select the value based on the CASE condition:

SELECT *, CASE WHEN a=b THEN acdc.myField ELSE bebop.myField END AS myField
  FROM abba 
       LEFT JOIN acdc ON abba.id=acdc.id 
       LEFT JOIN bebop ON abba.id=bebop.id

Answer 2

If the query is non-dynamic then no since case statement only returns a literal value such as string, int, or other data-types.

But there is a work-around - dynamic sql. You treat the entire statement as a string. Then you can work your cases inside the string.

For example:

declare @query varchar(1000)
set @query = 'select * from abba '+
                case when @a=@b then 'join acdc on abba.id=acdc.id'
                     when @b=@c then 'join bebop on abba.id=bebop.id'
                end
exec(@query)

But ofcourse, your a=b & b=c must be some sql variable.