SQL> select * from dept ,dept e;
DEPTNO DNAME LOC DEPTNO DNAME LOC
------ -------------- ------------- ------ -------------- -------------
10 ACCOUNTING NEW YORK 10 ACCOUNTING NEW YORK
10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS
10 ACCOUNTING NEW YORK 30 SALES CHICAGO
10 ACCOUNTING NEW YORK 40 OPERATIONS BOSTON
20 RESEARCH DALLAS 10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS 20 RESEARCH DALLAS
20 RESEARCH DALLAS 30 SALES CHICAGO
20 RESEARCH DALLAS 40 OPERATIONS BOSTON
30 SALES CHICAGO 10 ACCOUNTING NEW YORK
30 SALES CHICAGO 20 RESEARCH DALLAS
30 SALES CHICAGO 30 SALES CHICAGO
30 SALES CHICAGO 40 OPERATIONS BOSTON
40 OPERATIONS BOSTON 10 ACCOUNTING NEW YORK
40 OPERATIONS BOSTON 20 RESEARCH DALLAS
40 OPERATIONS BOSTON 30 SALES CHICAGO
40 OPERATIONS BOSTON 40 OPERATIONS BOSTON
16 rows selected
为什么表连接后,左右会显示4个ACCOUNTING,RESEARCH,SALES , OPERATIONS而右面则不一样
DEPTNO DNAME LOC DEPTNO DNAME LOC
------ -------------- ------------- ------ -------------- -------------
10 ACCOUNTING NEW YORK 10 ACCOUNTING NEW YORK
10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS
10 ACCOUNTING NEW YORK 30 SALES CHICAGO
10 ACCOUNTING NEW YORK 40 OPERATIONS BOSTON
20 RESEARCH DALLAS 10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS 20 RESEARCH DALLAS
20 RESEARCH DALLAS 30 SALES CHICAGO
20 RESEARCH DALLAS 40 OPERATIONS BOSTON
30 SALES CHICAGO 10 ACCOUNTING NEW YORK
30 SALES CHICAGO 20 RESEARCH DALLAS
30 SALES CHICAGO 30 SALES CHICAGO
30 SALES CHICAGO 40 OPERATIONS BOSTON
40 OPERATIONS BOSTON 10 ACCOUNTING NEW YORK
40 OPERATIONS BOSTON 20 RESEARCH DALLAS
40 OPERATIONS BOSTON 30 SALES CHICAGO
40 OPERATIONS BOSTON 40 OPERATIONS BOSTON
16 rows selected
为什么表连接后,左右会显示4个ACCOUNTING,RESEARCH,SALES , OPERATIONS而右面则不一样
select * from a,b;
如果a表记录数是m,b表记录数是n,则select查询的记录数是:m*n
dept有4条记录,两个dept笛卡尔连接,则4*4=16条记录右边一样,只是自然顺序按第一个表作为驱动表,按其它字段排序看看
select * from dept ,dept e order by e.DEPTNO;