集合運(yùn)算符

集合運(yùn)算符查詢組合兩個(gè)組件查詢的結(jié)果到一個(gè)結(jié)果，包含集合運(yùn)算符的查詢稱為復(fù)合查詢。SQL集合運(yùn)算符如下：

集合運(yùn)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> 查詢10和20號(hào)部門的員工

SQL> 1. select * from emp where deptno in (10,20);
SQL> 2. select * from emp where deptno=10 or deptno=20;
SQL> 3. 集合運(yùn)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> 注意的問(wèn)題

SQL> 1. 參與運(yùn)算的各個(gè)集合必須列數(shù)相同，且類型一致
SQL> 2. 采用第一個(gè)集合作為最后的表頭
SQL> 3. order by永遠(yuǎn)在最后
SQL> 4. 括號(hào)

SQL> select deptno,job,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),to_char(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

測(cè)試集合運(yùn)算和聚合函數(shù)的效率

SQL> --打開(kāi)SQL執(zhí)行時(shí)間的開(kāi)關(guān)
SQL> set timing on
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 ***已用時(shí)間: 00: 00: 00.02***

SQL> select deptno,job,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),to_char(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 ***已用時(shí)間: 00: 00: 00.08***

//關(guān)閉時(shí)間開(kāi)關(guān)
SQL> set timing off

結(jié)論：盡量少使用集合運(yùn)算。

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