select count( t1.search_terms) + sum(t2.count) from table1 t1 left join table2 t2 on (t1.search_engine = t2.search_engine) where t1.search_engine = 'Yahoo';
(SELECT count( search_terms ) FROM `table1` WHERE search_engine = 'Yahoo') + (SELECT SUM(count) FROM `table2` WHERE search_engine = 'Yahoo') AS summary