集合运算符
集合运算符查询组合两个组件查询的结果到一个结果，包含集合运算符的查询称为复合查询。sql集合运算符如下：
集合运算符
UNION：All distinct rows selected by either query
UNION ALL：All rows selected by either query,including all duplicates
INTERSECT：All distinct rows selected by both queries
MINUS：All distinct rows selected by the first query but not the second
sql> 1. select * from emp where deptno in (10,20);
sql> 2. select * from emp where deptno=10 or deptno=20;
sql> 3. 集合运算
sql> select * from emp where deptno=10
sql> 加上
sql> select * from emp where deptno=20
sql> select * from emp where deptno=10
2 union
3 select * from emp where deptno=20;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- -------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-12月-80 800 20
7566 JONES MANAGER 7839 02-4月 -81 2975 20
7782 CLARK MANAGER 7839 09-6月 -81 2450 10
7788 SCOTT ANALYST 7566 19-4月 -87 3000 20
7839 KING PRESIDENT 17-11月-81 5000 10
7876 ADAMS CLERK 7788 23-5月 -87 1100 20
7902 FORD ANALYST 7566 03-12月-81 3000 20
7934 MILLER CLERK 7782 23-1月 -82 1300 10
sql> select deptno,job,sum(sal) from emp group by rollup(deptno,job);
DEPTNO JOB SUM(SAL)
---------- --------- ----------
10 CLERK 1300
10 MANAGER 2450
10 PRESIDENT 5000
10 8750
20 CLERK 1900
20 ANALYST 6000
20 MANAGER 2975
20 10875
30 CLERK 950
30 MANAGER 2850
30 SALESMAN 5600
DEPTNO JOB SUM(SAL)
---------- --------- ----------
30 9400
29025
sql> 注意的问题
sql> 1. 参与运算的各个集合必须列数相同，且类型一致
sql> 2. 采用第一个集合作为最后的表头
sql> 3. order by永远在最后
sql> 4. 括号
sql> select deptno,sum(sal) from emp group by deptno,job
2 union
3 select deptno,to_char(null),sum(sal) from emp group by deptno
4 union
5 select to_number(null),sum(sal) from emp;
DEPTNO JOB SUM(SAL)
---------- --------- ----------
10 CLERK 1300
10 MANAGER 2450
10 PRESIDENT 5000
10 8750
20 ANALYST 6000
20 CLERK 1900
20 MANAGER 2975
20 10875
30 CLERK 950
30 MANAGER 2850
30 SALESMAN 5600
DEPTNO JOB SUM(SAL)
---------- --------- ----------
30 9400
29025
测试集合运算和聚合函数的效率
sql> –打开sql执行时间的开关
sql> set timing on
sql> select deptno,job);
DEPTNO JOB SUM(SAL)
---------- --------- ----------
10 CLERK 1300
10 MANAGER 2450
10 PRESIDENT 5000
10 8750
20 CLERK 1900
20 ANALYST 6000
20 MANAGER 2975
20 10875
30 CLERK 950
30 MANAGER 2850
30 SALESMAN 5600
DEPTNO JOB SUM(SAL)
---------- --------- ----------
30 9400
29025 ***已用时间: 00: 00: 00.02***
sql> select deptno,sum(sal) from emp;
DEPTNO JOB SUM(SAL)
---------- --------- ----------
10 CLERK 1300
10 MANAGER 2450
10 PRESIDENT 5000
10 8750
20 ANALYST 6000
20 CLERK 1900
20 MANAGER 2975
20 10875
30 CLERK 950
30 MANAGER 2850
30 SALESMAN 5600
DEPTNO JOB SUM(SAL)
---------- --------- ----------
30 9400
29025
***已用时间: 00: 00: 00.08***
结论：尽量少使用集合运算。
