oracle 多条查询问题，求大神

select * from test t1 where t1.row_id in
(select row_id from a)假如select row_id from a 得到的是空集合， 我想执行另外一条sql 怎么实现 就是说如果a表数据为空 我查另外一张表select * from test t1 where t1.row_id in
(select row_id from b)