sybase - values from one table that aren't on another, on opposite ends of a 3-table join

Answer 1

How about something like this:

  SELECT DISTINCT sign_pk
    FROM table_a
   WHERE sign_pk NOT IN
    (   
      SELECT DISTINCT front_sign sign
        FROM table_c
       UNION
      SELECT DISTINCT rear_sign sign
       FROM  table_c
    )

Answer 2

ANSI outer join is your friend here. *= has dodgy semantics and should be avoided

select distinct a.sign_pk, a.company_pk
from a join b on a.company_pk = b.company_pk 
left outer join c on b.company_location = c.company_location 
                  and (a.sign_pk = c.front_sign or a.sign_pk = c.back_sign) 
where c.company_location is null

Note that the where clause is a filter on the rows returned by the join, so it says "do the joins, but give me only the rows that didn't to join to c"

Outer join is almost always faster than NOT EXISTS and NOT IN