项目中遇到的一个动态查询

需求： sql 伪代码：select * from users where user_id in (select id from 如果参数flag=1 ，那么表名字为table1 ;如果参数flag=2，那么table的名字就为table2) 转换为2个sql即：如果flag=1，sql为：select * from users where user_id in (select id from table1); 如果flag=2，sql为：select * from users where user_id in (select id from table2);

实现： select * from users where user_id in
( select id from
( select t1.id, 1 flag from table1 union all select t2.id, 2 flag from table2
)where flag = decode (&flag,1,1,2,2)
);