If select true return select in mysql

Why not working this query (Undefined @param in true) :

    SELECT 
    c.ID ,
    IF((SELECT b.f1 into @param from db.tbl2 as b Where b.f2 = c.f2 AND b.f3 = c.f3),@param,c.off)  as 'off' 
    FROM db.tbl1 as c