【SQL】多表查询

多表查询，即查询可以从两个或多个表中获取数据。在Oracle中，有两种类型的连接格式：ANSI SQL连接格式和Oracle特有的连接格式。Oracle建议采用符合ANSI标准的连接格式。

1、内连接

（1）Oracle中特有的连接语法为：

SELECT table1.column,table2.column

FROM table1,table2

WHERE table1.column1=table2.column2;

如：

SQL> SELECT e.empno,e.ename,e.job,e.hiredate,e.sal,d.dname

2 FROM emp e ,dept d

3 WHERE e.deptno=d.deptno;

EMPNO ENAME JOB HIREDATE SAL DNAME

---------- ---------- --------- ------------------- ---------- --------------

7782 CLARK MANAGER 1981-06-09 00:00:00 2450 ACCOUNTING

7839 KING PRESIDENT 1981-11-17 00:00:00 5000 ACCOUNTING

7934 MILLER CLERK 1982-01-23 00:00:00 1300 ACCOUNTING

7566 JONES MANAGER 1981-04-02 00:00:00 2975 RESEARCH

7788 SCOTT ANALYST 1987-04-19 00:00:00 3000 RESEARCH

7876 ADAMS CLERK 1987-05-23 00:00:00 1100 RESEARCH

7902 FORD ANALYST 1981-12-03 00:00:00 3000 RESEARCH

7844 TURNER SALESMAN 1981-09-08 00:00:00 1500 SALES

7521 WARD SALESMAN 1981-02-22 00:00:00 1250 SALES

7499 ALLEN SALESMAN 1981-02-20 00:00:00 1600 SALES

7900 JAMES CLERK 1981-12-03 00:00:00 950 SALES

7698 BLAKE MANAGER 1981-05-01 00:00:00 2850 SALES

7654 MARTIN SALESMAN 1981-09-28 00:00:00 1250 SALES

13 rows selected.

（2）ANSI SQL的标准内连接语法为：

SELECT table1.column,table2.column

FROM table1 INNER JOIN table2

ON table1.column1=table2.column2;

如：

SQL> SELECT e.empno,e.ename,e.job,e.hiredate,e.sal,d.dname

2 FROM emp e INNER JOIN dept d

3 ON e.deptno=d.deptno;

EMPNO ENAME JOB HIREDATE SAL DNAME

---------- ---------- --------- ------------------- ---------- --------------

7782 CLARK MANAGER 1981-06-09 00:00:00 2450 ACCOUNTING

7839 KING PRESIDENT 1981-11-17 00:00:00 5000 ACCOUNTING

7934 MILLER CLERK 1982-01-23 00:00:00 1300 ACCOUNTING

7566 JONES MANAGER 1981-04-02 00:00:00 2975 RESEARCH

7788 SCOTT ANALYST 1987-04-19 00:00:00 3000 RESEARCH

7876 ADAMS CLERK 1987-05-23 00:00:00 1100 RESEARCH

7902 FORD ANALYST 1981-12-03 00:00:00 3000 RESEARCH

7844 TURNER SALESMAN 1981-09-08 00:00:00 1500 SALES

7521 WARD SALESMAN 1981-02-22 00:00:00 1250 SALES

7499 ALLEN SALESMAN 1981-02-20 00:00:00 1600 SALES

7900 JAMES CLERK 1981-12-03 00:00:00 950 SALES

7698 BLAKE MANAGER 1981-05-01 00:00:00 2850 SALES

7654 MARTIN SALESMAN 1981-09-28 00:00:00 1250 SALES

13 rows selected.

2、外连接

（1）Oracle中特有的连接语法为：

--右外连接

SELECT table1.column,table2.column

FROM table1, table2

WHERE table1.column1(+)=table2.column2;

--左外连接

SELECT table1.column,table2.column

FROM table1, table2

WHERE table1.column1=table2.column2(+);

注意：

右外连接：当（+）符号在等号左边，将返回table2表中的所有数据。

左外连接：当（+）符号在等号右边，将返回table1表中的所有数据。

等号两边不能同时都加（+）符号

（2）ANSI SQL标准中的连接、右连接和全连接语法为：

SELECT table1.column,table2.column

FROM table1

[LEFT|RIGHT|FULL OUTER JOIN table2

ON (table1.column=table2.column)];

3、交叉连接（笛卡尔积）

如果在进行连接的时候没有指定连接条件，则会产生笛卡尔积，返回的记录数是两个表中记录数的乘积。

4、自然连接

前提要求：两个表中有相同名称、相同数据类型的列。如果两个标的列名相同而数据类型不相同，也不能进行自然连接。自然连接使用NATRUAL JOIN关键字

如：

SQL> SELECT e.empno,e.ename,e.job,e.hiredate,e.sal,d.dname

2 FROM emp e NATURAL JOIN dept d;

EMPNO ENAME JOB HIREDATE SAL DNAME

---------- ---------- --------- ------------------- ---------- --------------

7782 CLARK MANAGER 1981-06-09 00:00:00 2450 ACCOUNTING

7839 KING PRESIDENT 1981-11-17 00:00:00 5000 ACCOUNTING

7934 MILLER CLERK 1982-01-23 00:00:00 1300 ACCOUNTING

7566 JONES MANAGER 1981-04-02 00:00:00 2975 RESEARCH

7788 SCOTT ANALYST 1987-04-19 00:00:00 3000 RESEARCH

7876 ADAMS CLERK 1987-05-23 00:00:00 1100 RESEARCH

7902 FORD ANALYST 1981-12-03 00:00:00 3000 RESEARCH

7844 TURNER SALESMAN 1981-09-08 00:00:00 1500 SALES

7521 WARD SALESMAN 1981-02-22 00:00:00 1250 SALES

7499 ALLEN SALESMAN 1981-02-20 00:00:00 1600 SALES

7900 JAMES CLERK 1981-12-03 00:00:00 950 SALES

7698 BLAKE MANAGER 1981-05-01 00:00:00 2850 SALES

7654 MARTIN SALESMAN 1981-09-28 00:00:00 1250 SALES

13 rows selected.