集合操作符
UNION/UNION ALL
INTERSECT
MINUS
Union All不排序，不去重，其余均升序且去重。
create table e1 as select * from emp where deptno in (10,20);
create table e2 as select * from emp where deptno in (20,30);
select * from e1 union select * from e2 order by 8
select * from e1 union all select * from e2 order by 8
select * from e1 intersect select * from e2 order by 1 desc
select * from e1 minus select * from e2 order by 1 desc
SELECT location_id, department_name "Department",
TO_CHAR(NULL) "Warehouse location"
FROM departments
UNION
SELECT location_id, TO_CHAR(NULL) "Department",
state_province
FROM locations;
SELECT employee_id, job_id,salary
FROM employees
UNION
SELECT employee_id, job_id,0
FROM job_history;
