sql query selecting from two tables - return results from one table if there are none in the other table
<p>i am running this SQL query:</p>

<pre><code>SELECT a.retail, b.cost 
from call_costs a, call_costs_custom b 
WHERE a.sequence = b.parent 
 AND a.sequence = '15684' 
 AND b.customer_seq = '124'
</code></pre>

<p>which returns both <code>a.retail</code> and <code>b.cost</code> if the row exists in <code>call_costs_custom</code> but if the row does not exist, i want to show just <code>a.retail</code> using the <code>WHERE</code> clauses for <code>a. (call_costs)</code></p>