目錄一、內(nèi)連接（表1 inner join 表2 on）1、顯示SMITH的名字和部門名稱二、外連接1、左外連接（表名1 left join 表名2 on）2、右外連接（表名1 right join 表名2）一、內(nèi)連接（表1 inner join 表2 on）

內(nèi)連接就是利用where子句對兩種表形成的笛卡爾積進(jìn)行篩選，之前博客寫的查詢都是內(nèi)連接，也是在開發(fā)過程中使用的最多的連接查詢。

語法：

select 字段 from 表1 inner join 表2 on 連接條件 and 其他條件;--或者select 字段 from 表1 inner join 表2 on 連接條件 where 條件 and 條件;1、顯示SMITH的名字和部門名稱--兩張數(shù)據(jù)表mysql> select* from emp;+--------+--------+-----------+------+---------------------+---------+---------+--------+| empno | ename | job | mgr | hiredate | sal | comm | deptno |+--------+--------+-----------+------+---------------------+---------+---------+--------+| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 || 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 || 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 || 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 || 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 || 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 || 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 || 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 || 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 || 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 || 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 || 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 || 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 || 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |+--------+--------+-----------+------+---------------------+---------+---------+--------+14 rows in set (0.07 sec)mysql> select* from dept;+--------+------------+----------+| deptno | dname | loc |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+4 rows in set (0.00 sec)

之前的寫法：

mysql> select ename,dname from emp,dept where emp.deptno=dept.deptno and ename='SMITH';+-------+----------+| ename | dname |+-------+----------+| SMITH | RESEARCH |+-------+----------+1 row in set (0.01 sec)

標(biāo)準(zhǔn)的內(nèi)連接的寫法：

mysql> select ename,dname from emp inner join dept on emp.deptno=dept.deptno and ename='SMITH';+-------+----------+| ename | dname |+-------+----------+| SMITH | RESEARCH |+-------+----------+1 row in set (0.00 sec)mysql> select ename,dname from emp inner join dept on emp.deptno=dept.deptno where ename='SMITH';+-------+----------+| ename | dname |+-------+----------+| SMITH | RESEARCH |+-------+----------+1 row in set (0.00 sec)二、外連接

外連接分為左外連接和右外連接。

1、左外連接（表名1 left join 表名2 on）

如果聯(lián)合查詢，左側(cè)的表完全顯示就是左外連接。

語法：

select 字段名 from 表名1 left join 表名2 on 連接條件;

整兩張表：

-- 學(xué)生表create table stu (id int, name varchar(30)); insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');--成績表create table exam (id int, grade int);insert into exam values(1, 56),(2,76),(11, 8);

1.1查詢所有學(xué)生的成績，如果這個(gè)學(xué)生沒有成績，也要將學(xué)生的個(gè)人信息顯示出來

學(xué)生表和成績表的id并不是一一對應(yīng)的，如果這里用內(nèi)連接，未在兩個(gè)表中出現(xiàn)的id字段將不會被篩選出來：

mysql> select* from stu inner join exam on stu.id=exam.id;+------+------+------+-------+| id | name | id | grade |+------+------+------+-------+| 1 | jack | 1 | 56 || 2 | tom | 2 | 76 |+------+------+------+-------+2 rows in set (0.00 sec)

所以此處需要使用左外連接：

mysql> select* from stu left join exam on stu.id=exam.id;+------+------+------+-------+| id | name | id | grade |+------+------+------+-------+| 1 | jack | 1 | 56 || 2 | tom | 2 | 76 || 3 | kity | NULL | NULL || 4 | nono | NULL | NULL |+------+------+------+-------+4 rows in set (0.00 sec)2、右外連接（表名1 right join 表名2）

如果聯(lián)合查詢，右側(cè)的表完全顯示我們就說是右外連接。

語法：

select 字段 from 表名1 right join 表名2 on 連接條件;

2.1對stu表和exam表聯(lián)合查詢，把所有的成績都顯示出來，即使這個(gè)成績沒有學(xué)生與它對應(yīng)，也要顯示出來

mysql> select exam.id,name,grade from stu right join exam on stu.id=exam.id;+------+------+-------+| id | name | grade |+------+------+-------+| 1 | jack | 56 || 2 | tom | 76 || 11 | NULL | 8 |+------+------+-------+3 rows in set (0.00 sec)

2.2列出部門名稱和這些部門的員工信息，同時(shí)列出沒有員工的部門

mysql> select * from emp right join dept on emp.deptno=dept.deptno order by emp.deptno asc;+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc |+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+| NULL | NULL | NULL | NULL | NULL| NULL | NULL | NULL | 40 | OPERATIONS | BOSTON || 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK || 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK || 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK || 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS || 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS || 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS || 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS || 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS || 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO || 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO || 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO || 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO || 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO || 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+15 rows in set (0.00 sec)

以上就是詳解MySQL中表的內(nèi)外連接的詳細(xì)內(nèi)容，更多關(guān)于MySQL內(nèi)外連接的資料請關(guān)注好吧啦網(wǎng)其它相關(guān)文章！