Add reverse IN clause to db API
That's (arguably) non-standard, so you'll need a where. 

For example,

$query = db_select('foo');
$query-&gt;where(':val IN ({table1}.col1, {table2}.col2)', array(':val' =&gt; $user-&gt;uid));


will get you

SELECT 
FROM 
{foo} foo
WHERE  (:val IN ({table1}.col1, {table2}.col2)) 


(with :val replaced by $user-&gt;uid).