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MySQL多表查询 建表与数据准备 #建表 create table department( #部门表 id int, name varchar(20) ); create table employee( #员工表 id int primary key auto_increment, name varchar(20), sex enum('male','female') not null default 'male', age int, dep_id int ); #插入数据 insert into department values (200,'技术'), (201,'人力资源'), (202,'销售'), (203,'运营'); insert into employee(name,sex,age,dep_id) values ('daxia','female',18,200), ('anwen','male',23,201), ('wudi','male',38,201), ('yage','female',23,202), ('dafei','male',18,200), ('jingyu','female',18,204) ; #查看表结构和数据 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ mysql> desc employee; +--------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(11) | YES | | NULL | | | dep_id | int(11) | YES | | NULL | | +--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技术 | | 201 | 人力资源 | | 202 | 销售 | | 203 | 运营 | +------+--------------+ mysql> select * from employee; +----+------------+--------+------+--------+ | id | name | sex | age | dep_id | +----+------------+--------+------+--------+ | 1 | daxia | female | 18 | 200 | | 2 | anwen | male | 23 | 201 | | 3 | wudi | male | 38 | 201 | | 4 | yage | female | 23 | 202 | | 5 | dafei | male | 18 | 200 | | 6 | jingyu | female | 18 | 204 | +----+------------+--------+------+--------+ 多表连接查询 #重点：外链接语法 SELECT 字段列表 FROM 表1 INNER|LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段;

1 交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee,department; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | daxia | female | 18 | 200 | 200 | 技术 | | 1 | daxia | female | 18 | 200 | 201 | 人力资源 | | 1 | daxia | female | 18 | 200 | 202 | 销售 | | 1 | daxia | female | 18 | 200 | 203 | 运营 | | 2 | anwen | male | 48 | 201 | 200 | 技术 | | 2 | anwen | male | 48 | 201 | 201 | 人力资源 | | 2 | anwen | male | 48 | 201 | 202 | 销售 | | 2 | anwen | male | 48 | 201 | 203 | 运营 | | 3 | wudi | male | 38 | 201 | 200 | 技术 | | 3 | wudi | male | 38 | 201 | 201 | 人力资源 | | 3 | wudi | male | 38 | 201 | 202 | 销售 | | 3 | wudi | male | 38 | 201 | 203 | 运营 | | 4 | yage | female | 28 | 202 | 200 | 技术 | | 4 | yage | female | 28 | 202 | 201 | 人力资源 | | 4 | yage | female | 28 | 202 | 202 | 销售 | | 4 | yage | female | 28 | 202 | 203 | 运营 | | 5 | dafei | male | 18 | 200 | 200 | 技术 | | 5 | dafei | male | 18 | 200 | 201 | 人力资源 | | 5 | dafei | male | 18 | 200 | 202 | 销售 | | 5 | dafei | male | 18 | 200 | 203 | 运营 | | 6 | jingyu | female | 18 | 204 | 200 | 技术 | | 6 | jingyu | female | 18 | 204 | 201 | 人力资源 | | 6 | jingyu | female | 18 | 204 | 202 | 销售 | | 6 | jingyu | female | 18 | 204 | 203 | 运营 | +----+------------+--------+------+--------+------+--------------+

2 内连接：只连接匹配的行

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果 #department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; +----+-----------+------+--------+--------------+ | id | name | age | sex | name | +----+-----------+------+--------+--------------+ | 1 | daxia | 18 | female | 技术 | | 2 | anwen | 48 | male | 人力资源 | | 3 | wudi | 38 | male | 人力资源 | | 4 | yage | 28 | female | 销售 | | 5 | dafei | 18 | male | 技术 | +----+-----------+------+--------+--------------+ #上述sql等同于 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;