HELP with a Query

Hi,
I need some help with a query.
I use a reference table to lookup some codes. Everything works well if the reference table contains a reference but the
queryfails if no reference fails (Z.ref_code=123456).  
What I need to is either return 0 or null into Z.prim_exch if no reference can be found. Whats the best way to do that
?I tried a sub query but also did not succeed. 


SELECT A.sec_code,A.exch_code,A.price_date,Z.prim_exch,
   CASE
     WHEN A.exch_code = Z.prim_exch THEN 1
     ELSE 0
   END AS is_prim
FROM price_hist AS A, references AS Z
WHERE   A.price_date <= '20040219' AND A.sec_code=123456 AND Z.ref_code=123456 LIMIT 10;

Thanks for any help.
Alex