oracle笛卡尔积与等值连接

要显示雇员名，雇员工资及所在部门的名字

第一步：select * from emp a1,dept a2

没有连接条件的返回结果会形成笛卡尔积，结果如下。很多生成的记录是没有意义的。

	EMPNO	ENAME	JOB	MGR	HIREDATE	SAL	COMM	DEPTNO	DETPNO	DNAME	LOC
1	7369	SMITH	CLERK	7902	1980/12/17	800		20	10	ACCOUNTING	NEW YORK
2	7499	ALLEN	SALESMAN	7698	1981/2/20	1600	300	30	10	ACCOUNTING	NEW YORK
3	7521	WARD	SALESMAN	7698	1981/2/22	1250	500	30	10	ACCOUNTING	NEW YORK
4	7566	JONES	MANAGER	7839	1981/4/2	2975		20	10	ACCOUNTING	NEW YORK
5	7654	MARTIN	SALESMAN	7698	1981/9/28	1250	1400	30	10	ACCOUNTING	NEW YORK
6	7698	BLAKE	MANAGER	7839	1981/5/1	2850		30	10	ACCOUNTING	NEW YORK
7	7782	CLARK	MANAGER	7839	1981/6/9	2450		10	10	ACCOUNTING	NEW YORK
8	7788	SCOTT	ANALYST	7566	1987/4/19	1500		20	10	ACCOUNTING	NEW YORK
9	7839	KING	PRESIDENT		1981/11/17	5000		10	10	ACCOUNTING	NEW YORK
10	7844	TURNER	SALESMAN	7698	1981/9/8	1500	0	30	10	ACCOUNTING	NEW YORK
11	7876	ADAMS	CLERK	7788	1987/5/23	1100		20	10	ACCOUNTING	NEW YORK
12	7900	JAMES	CLERK	7698	1981/12/3	950		30	10	ACCOUNTING	NEW YORK
13	7902	FORD	ANALYST	7566	1981/12/3	3000		20	10	ACCOUNTING	NEW YORK
14	7934	MILLER	CLERK	7782	1982/1/23	1300		10	10	ACCOUNTING	NEW YORK
15	7369	SMITH	CLERK	7902	1980/12/17	800		20	20	RESEARCH	DALLAS
16	7499	ALLEN	SALESMAN	7698	1981/2/20	1600	300	30	20	RESEARCH	DALLAS
17	7521	WARD	SALESMAN	7698	1981/2/22	1250	500	30	20	RESEARCH	DALLAS
18	7566	JONES	MANAGER	7839	1981/4/2	2975		20	20	RESEARCH	DALLAS
19	7654	MARTIN	SALESMAN	7698	1981/9/28	1250	1400	30	20	RESEARCH	DALLAS
20	7698	BLAKE	MANAGER	7839	1981/5/1	2850		30	20	RESEARCH	DALLAS
21	7782	CLARK	MANAGER	7839	1981/6/9	2450		10	20	RESEARCH	DALLAS
22	7788	SCOTT	ANALYST	7566	1987/4/19	1500		20	20	RESEARCH	DALLAS
23	7839	KING	PRESIDENT		1981/11/17	5000		10	20	RESEARCH	DALLAS
24	7844	TURNER	SALESMAN	7698	1981/9/8	1500	0	30	20	RESEARCH	DALLAS
25	7876	ADAMS	CLERK	7788	1987/5/23	1100		20	20	RESEARCH	DALLAS
26	7900	JAMES	CLERK	7698	1981/12/3	950		30	20	RESEARCH	DALLAS
27	7902	FORD	ANALYST	7566	1981/12/3	3000		20	20	RESEARCH	DALLAS
28	7934	MILLER	CLERK	7782	1982/1/23	1300		10	20	RESEARCH	DALLAS
29	7369	SMITH	CLERK	7902	1980/12/17	800		20	30	SALES	CHICAGO
30	7499	ALLEN	SALESMAN	7698	1981/2/20	1600	300	30	30	SALES	CHICAGO
31	7521	WARD	SALESMAN	7698	1981/2/22	1250	500	30	30	SALES	CHICAGO
32	7566	JONES	MANAGER	7839	1981/4/2	2975		20	30	SALES	CHICAGO
33	7654	MARTIN	SALESMAN	7698	1981/9/28	1250	1400	30	30	SALES	CHICAGO
34	7698	BLAKE	MANAGER	7839	1981/5/1	2850		30	30	SALES	CHICAGO
35	7782	CLARK	MANAGER	7839	1981/6/9	2450		10	30	SALES	CHICAGO
36	7788	SCOTT	ANALYST	7566	1987/4/19	1500		20	30	SALES	CHICAGO
37	7839	KING	PRESIDENT		1981/11/17	5000		10	30	SALES	CHICAGO
38	7844	TURNER	SALESMAN	7698	1981/9/8	1500	0	30	30	SALES	CHICAGO
39	7876	ADAMS	CLERK	7788	1987/5/23	1100		20	30	SALES	CHICAGO
40	7900	JAMES	CLERK	7698	1981/12/3	950		30	30	SALES	CHICAGO
41	7902	FORD	ANALYST	7566	1981/12/3	3000		20	30	SALES	CHICAGO
42	7934	MILLER	CLERK	7782	1982/1/23	1300		10	30	SALES	CHICAGO
43	7369	SMITH	CLERK	7902	1980/12/17	800		20	40	OPERATIONS	BOSTON
44	7499	ALLEN	SALESMAN	7698	1981/2/20	1600	300	30	40	OPERATIONS	BOSTON
45	7521	WARD	SALESMAN	7698	1981/2/22	1250	500	30	40	OPERATIONS	BOSTON
46	7566	JONES	MANAGER	7839	1981/4/2	2975		20	40	OPERATIONS	BOSTON
47	7654	MARTIN	SALESMAN	7698	1981/9/28	1250	1400	30	40	OPERATIONS	BOSTON
48	7698	BLAKE	MANAGER	7839	1981/5/1	2850		30	40	OPERATIONS	BOSTON
49	7782	CLARK	MANAGER	7839	1981/6/9	2450		10	40	OPERATIONS	BOSTON
50	7788	SCOTT	ANALYST	7566	1987/4/19	1500		20	40	OPERATIONS	BOSTON
51	7839	KING	PRESIDENT		1981/11/17	5000		10	40	OPERATIONS	BOSTON
52	7844	TURNER	SALESMAN	7698	1981/9/8	1500	0	30	40	OPERATIONS	BOSTON
53	7876	ADAMS	CLERK	7788	1987/5/23	1100		20	40	OPERATIONS	BOSTON
54	7900	JAMES	CLERK	7698	1981/12/3	950		30	40	OPERATIONS	BOSTON
55	7902	FORD	ANALYST	7566	1981/12/3	3000		20	40	OPERATIONS	BOSTON
56	7934	MILLER	CLERK	7782	1982/1/23	1300		10	40	OPERATIONS	BOSTON

第二步，等值连接消除笛卡尔积

    

select * from emp a1,dept a2 where a1.deptno=a2.deptno;

第三步，选取列

select a1.ename,a1.sal,a2.dname from emp a1,dept a2 where a1.deptno=a2.deptno;