实现多表的简单连接时，如果仅仅通过select子句和from子句连接多个表，那么查询的结果将是一个通过笛卡儿积所生成的表。
所谓的笛卡儿积所生成的表，就是一个基本表中每一行与另一个基本表中的每一行连接在一起所生成的表，查询结果的行数是两个基本表的行数的积。
如下，使用oracle中的emp和dept两个表连接查询：
SQL> select * from emp,dept;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DEPTNO DNAME LOC
------ --------- --------- ----- ---------- ------ ----- ------ ------ ------------ -------------
7369 SMITH CLERK 7902 17-12月-80 800 20 10 ACCOUNTING NEW YORK
7499 ALLEN SALESMAN 7698 20-2月 -81 1600 300 30 10 ACCOUNTING NEW YORK
7521 WARD SALESMAN 7698 22-2月 -81 1250 500 30 10 ACCOUNTING NEW YORK
7566 JONES MANAGER 7839 02-4月 -81 2975 20 10 ACCOUNTING NEW YORK
7654 MARTIN SALESMAN 7698 28-9月 -81 1250 1400 30 10 ACCOUNTING NEW YORK
7698 BLAKE MANAGER 7839 01-5月 -81 2850 30 10 ACCOUNTING NEW YORK
7782 CLARK MANAGER 7839 09-6月 -81 2450 10 10 ACCOUNTING NEW YORK
7788 SCOTT ANALYST 7566 24-1月 -87 3000 20 10 ACCOUNTING NEW YORK
7839 KING PRESIDENT 17-11月-81 5000 10 10 ACCOUNTING NEW YORK
7844 TURNER SALESMAN 7698 08-9月 -81 1500 0 30 10 ACCOUNTING NEW YORK
7876 ADAMS CLERK 7788 02-4月 -87 1100 20 10 ACCOUNTING NEW YORK
7900 JAMES CLERK 7698 03-12月-81 1800 30 10 ACCOUNTING NEW YORK
7902 FORD ANALYST 7566 03-12月-81 3000 20 10 ACCOUNTING NEW YORK
7934 MILLER CLERK 7782 23-1月 -82 1300 10 10 ACCOUNTING NEW YORK
7935 XIAOXUE MANAGER 7839 01-3月 -98 5000 500 20 10 ACCOUNTING NEW YORK
7369 SMITH CLERK 7902 17-12月-80 800 20 20 RESEARCH DALLAS
7499 ALLEN SALESMAN 7698 20-2月 -81 1600 300 30 20 RESEARCH DALLAS
7521 WARD SALESMAN 7698 22-2月 -81 1250 500 30 20 RESEARCH DALLAS
7566 JONES MANAGER 7839 02-4月 -81 2975 20 20 RESEARCH DALLAS
7654 MARTIN SALESMAN 7698 28-9月 -81 1250 1400 30 20 RESEARCH DALLAS
7698 BLAKE MANAGER 7839 01-5月 -81 2850 30 20 RESEARCH DALLAS
7782 CLARK MANAGER 7839 09-6月 -81 2450 10 20 RESEARCH DALLAS
7788 SCOTT ANALYST 7566 24-1月 -87 3000 20 20 RESEARCH DALLAS
7839 KING PRESIDENT 17-11月-81 5000 10 20 RESEARCH DALLAS
7844 TURNER SALESMAN 7698 08-9月 -81 1500 0 30 20 RESEARCH DALLAS
7876 ADAMS CLERK 7788 02-4月 -87 1100 20 20 RESEARCH DALLAS
7900 JAMES CLERK 7698 03-12月-81 1800 30 20 RESEARCH DALLAS
7902 FORD ANALYST 7566 03-12月-81 3000 20 20 RESEARCH DALLAS
7934 MILLER CLERK 7782 23-1月 -82 1300 10 20 RESEARCH DALLAS
7935 XIAOXUE MANAGER 7839 01-3月 -98 5000 500 20 20 RESEARCH DALLAS
7369 SMITH CLERK 7902 17-12月-80 800 20 30 SALES CHICAGO
7499 ALLEN SALESMAN 7698 20-2月 -81 1600 300 30 30 SALES CHICAGO
7521 WARD SALESMAN 7698 22-2月 -81 1250 500 30 30 SALES CHICAGO
7566 JONES MANAGER 7839 02-4月 -81 2975 20 30 SALES CHICAGO
7654 MARTIN SALESMAN 7698 28-9月 -81 1250 1400 30 30 SALES CHICAGO
7698 BLAKE MANAGER 7839 01-5月 -81 2850 30 30 SALES CHICAGO
7782 CLARK MANAGER 7839 09-6月 -81 2450 10 30 SALES CHICAGO
7788 SCOTT ANALYST 7566 24-1月 -87 3000 20 30 SALES CHICAGO
7839 KING PRESIDENT 17-11月-81 5000 10 30 SALES CHICAGO
7844 TURNER SALESMAN 7698 08-9月 -81 1500 0 30 30 SALES CHICAGO
7876 ADAMS CLERK 7788 02-4月 -87 1100 20 30 SALES CHICAGO
7900 JAMES CLERK 7698 03-12月-81 1800 30 30 SALES CHICAGO
7902 FORD ANALYST 7566 03-12月-81 3000 20 30 SALES CHICAGO
7934 MILLER CLERK 7782 23-1月 -82 1300 10 30 SALES CHICAGO
7935 XIAOXUE MANAGER 7839 01-3月 -98 5000 500 20 30 SALES CHICAGO
7369 SMITH CLERK 7902 17-12月-80 800 20 40 OPERATIONS BOSTON
7499 ALLEN SALESMAN 7698 20-2月 -81 1600 300 30 40 OPERATIONS BOSTON
7521 WARD SALESMAN 7698 22-2月 -81 1250 500 30 40 OPERATIONS BOSTON
7566 JONES MANAGER 7839 02-4月 -81 2975 20 40 OPERATIONS BOSTON
7654 MARTIN SALESMAN 7698 28-9月 -81 1250 1400 30 40 OPERATIO |