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詳解MySQL中表的內外連接
瀏覽：232日期：2023-08-28 20:27:10
目錄一、內連接（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、內連接（表1 inner join 表2 on）
內連接就是利用where子句對兩種表形成的笛卡爾積進行篩選，之前博客寫的查詢都是內連接，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--兩張數據表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)標準的內連接的寫法：
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）如果聯合查詢，左側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查詢所有學生的成績，如果這個學生沒有成績，也要將學生的個人信息顯示出來
學生表和成績表的id并不是一一對應的，如果這里用內連接，未在兩個表中出現的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）如果聯合查詢，右側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表聯合查詢，把所有的成績都顯示出來，即使這個成績沒有學生與它對應，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中表的內外連接的詳細內容，更多關于MySQL內外連接的資料請關注好吧啦網其它相關文章！
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