oracle 常用集合

1. union

2. union all

3. minus  等于 not exists  如下：
    select *  from test_employees e
       where not exists (select * from test_employees2 e2 where e.id = e2.id);

     select * from test_employees
     minus
    select * from test_employees2;
     
4. intersect  等于 exists

    select * from test_employees e
       where not exists (select * from test_employees2 e2 where e.id = e2.id);

    select * from test_employees
intersect
    select * from test_employees2;