詳解MySQL中表的內(nèi)外連接

更新時(shí)間：2023年06月25日 09:00:48 作者：蔣靈瑜的筆記本

這篇文章主要為大家介紹了MySQL中表的內(nèi)外連接的相關(guān)知識(shí)，文中通過示例代碼介紹的非常詳細(xì)，具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友們下面隨著小編來一起學(xué)習(xí)學(xué)習(xí)吧

一、內(nèi)連接（表1 inner join 表2 on）

內(nèi)連接就是利用where子句對(duì)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(duì)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(huì)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(duì)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(duì)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(qǐng)關(guān)注腳本之家其它相關(guān)文章！

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