Oracle数据库之集合运算

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> 查询10和20号部门的员工

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***

//关闭时间开关
SQL> set timing off

结论：尽量少使用集合运算。

（编辑：李大同）

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