文章目录

• 一、表的内连接
• 二、表的外连接
• • 1.左外连接
  • 2.右外连接

    一、表的内连接

    内连接实际上就是利用where子句对两种表形成的笛卡儿积进行筛选，我们前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询。

    语法：

    select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；
    

    --用前面的写法
    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)
    -- 用标准的内连接写法
    mysql> select ename,dname from emp inner join dept where emp.deptno = dept.deptno and ename='SMITH';
    +-------+----------+
    | ename | dname    |
    +-------+----------+
    | SMITH | RESEARCH |
    +-------+----------+
    1 row in set (0.00 sec)
    

    以下两种种写法是等价的

    select ename,dname from emp inner join dept on emp.deptno=dept.deptno where name='SMITH';
    select ename,dname from emp inner join dept on emp.deptno=dept.deptno and name='SMITH';
    

    二、表的外连接

    1.左外连接

    如果联合查询，左侧的表完全显示我们就说是左外连接。

    语法：

    select 字段名 from 表名1 left join 表名2 on 连接条件
    

    案例：

    – 建两张表–学生表和成绩表

    create table stu (id int, name varchar(30)); -- 学生表
    insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
    create table exam (id int, grade int); -- 成绩表
    insert into exam values(1, 56),(2,76),(11, 8);
    mysql> select * from stu;
    +------+------+
    | id   | name |
    +------+------+
    |    1 | jack |
    |    2 | tom  |
    |    3 | kity |
    |    4 | nono |
    +------+------+
    4 rows in set (0.00 sec)
    mysql> select * from exam;
    +------+-------+
    | id   | grade |
    +------+-------+
    |    1 |    56 |
    |    2 |    76 |
    |   11 |     8 |
    +------+-------+
    3 rows in set (0.01 sec)
    

    查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来

    当左边表和右边表没有匹配时，也会显示左边表的数据

    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.右外连接

    如果联合查询，右侧的表完全显示我们就说是右外连接。

    语法：

    select 字段 from 表名1 right join 表名2 on 连接条件;
    

    对stu表和exam表联合查询，把所有的成绩都显示出来，即使这个成绩没有学生与它对应，也要显示出来

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

    列出部门名称和这些部门的员工信息，同时列出没有员工的部门

    方法一：
    select dept.dname,emp.* from dept left join emp on emp.deptno=dept.deptno;
    方法二：
    select dept.dname,emp.* from emp right join dept on emp.deptno=dept.deptno;
    +------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
    | dname      | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
    +------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
    | RESEARCH   | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
    | SALES      | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
    | SALES      | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
    | RESEARCH   | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
    | SALES      | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
    | SALES      | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
    | ACCOUNTING | 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
    | RESEARCH   | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
    | ACCOUNTING | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
    | SALES      | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
    | RESEARCH   | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
    | SALES      | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
    | RESEARCH   | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
    | ACCOUNTING | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
    | OPERATIONS |   NULL | NULL   | NULL      | NULL | NULL                |    NULL |    NULL |   NULL |
    +------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
    

          |    NULL |    NULL |   NULL |
    

    ±-----------±-------±-------±----------±-----±--------------------±--------±--------±-------+

     
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