MySql - 多表查询
多表查询
- 1. 多表连接查询
- 2.符合条件连接查询
- 3. 子查询
数据准备
create table department( id int, name varchar(20) ); create table employee( 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 ('egon','male',18,200), ('alex','female',48,201), ('wupeiqi','male',38,201), ('yuanhao','female',28,202), ('nvshen','male',18,200), ('xiaomage','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 | | +-------+-------------+------+-----+---------+-------+ rows in set (0.19 sec) 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 | | +--------+-----------------------+------+-----+---------+----------------+ rows in set (0.01 sec) mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技术 | | 201 | 人力资源 | | 202 | 销售 | | 203 | 运营 | +------+--------------+ rows in set (0.02 sec) mysql> select * from employee; +----+----------+--------+------+--------+ | id | name | sex | age | dep_id | +----+----------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 201 | | 4 | yuanhao | female | 28 | 202 | | 5 | nvshen | male | 18 | 200 | | 6 | xiaomage | female | 18 | 204 | +----+----------+--------+------+--------+ rows in set (0.00 sec)
1. 多表连接查询
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;
1.1 一个概念 : 笛卡尔积
- 笛卡尔积
mysql> select * from employee2,department; +----+----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 1 | egon | male | 18 | 200 | 201 | 人力资源 | | 1 | egon | male | 18 | 200 | 202 | 销售 | | 1 | egon | male | 18 | 200 | 203 | 运营 | | 2 | alex | female | 48 | 201 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 2 | alex | female | 48 | 201 | 202 | 销售 | | 2 | alex | female | 48 | 201 | 203 | 运营 | | 3 | wupeiqi | male | 38 | 201 | 200 | 技术 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 202 | 销售 | | 3 | wupeiqi | male | 38 | 201 | 203 | 运营 | | 4 | yuanhao | female | 28 | 202 | 200 | 技术 | | 4 | yuanhao | female | 28 | 202 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 4 | yuanhao | female | 28 | 202 | 203 | 运营 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | | 5 | nvshen | male | 18 | 200 | 201 | 人力资源 | | 5 | nvshen | male | 18 | 200 | 202 | 销售 | | 5 | nvshen | male | 18 | 200 | 203 | 运营 | | 6 | xiaomage | female | 18 | 204 | 200 | 技术 | | 6 | xiaomage | female | 18 | 204 | 201 | 人力资源 | | 6 | xiaomage | female | 18 | 204 | 202 | 销售 | | 6 | xiaomage | female | 18 | 204 | 203 | 运营 | +----+----------+--------+------+--------+------+--------------+ 24 rows in set (0.11 sec)
- 符合条件查询
mysql> select * from employee,department where employee.dep_id = department.id; +----+---------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+---------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | +----+---------+--------+------+--------+------+--------------+ 5 rows in set (0.01 sec)
1.2 内连接
mysql> select * from employee inner join department on employee.dep_id = department.id; +----+---------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+---------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | +----+---------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec)
通过上表可以看出,内连接是找到两张表共有的部分,相当于利用条件从笛卡尔积结果中筛选出了匹配的结果
(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) -> 与上面的符合条件查询相同
1.3 左连接或右连接 (优先显示左表或者右表的全部记录)
- 左连接
mysql> select * from employee left join department on employee.dep_id = department.id; +----+----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | xiaomage | female | 18 | 204 | NULL | NULL | +----+----------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
- 右连接
mysql> select * from employee right join department on employee.dep_id = department.id; +------+---------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+---------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+---------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
1.4 全外连接 (显示两个表中的全部记录)
mysql> select * from employee left join department on employee.dep_id = department.id -> union -> select * from employee right join department on employee.dep_id = department.id; +------+----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | nvshen | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | xiaomage | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+----------+--------+------+--------+------+--------------+ 7 rows in set (0.01 sec)
2. 符合条件连接查询
- 找出年龄大于25岁的员工以及员工所在的部门
mysql> select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age>25; +---------+--------------+ | name | name | +---------+--------------+ | alex | 人力资源 | | wupeiqi | 人力资源 | | yuanhao | 销售 | +---------+--------------+ 3 rows in set (0.00 sec)
- 找出年龄大于25岁的员工以及员工所在的部门,并且以age字段的升序方式显示。
mysql> select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age>25 order by age asc; +---------+--------------+ | name | name | +---------+--------------+ | yuanhao | 销售 | | wupeiqi | 人力资源 | | alex | 人力资源 | +---------+--------------+ 3 rows in set (0.00 sec)
3. 子查询
3.1 带in 关键字的子查询(练习题)
- 查询平均年龄在25岁以上的部门的名称
mysql> select dep_id from employee group by dep_id having avg(age)>25; +--------+ | dep_id | +--------+ | 201 | | 202 | +--------+ 2 rows in set (0.38 sec) mysql> select * from department where id in (select dep_id from employee group by dep_id having avg(age)>25); +------+--------------+ | id | name | +------+--------------+ | 201 | 人力资源 | | 202 | 销售 | +------+--------------+ 2 rows in set (0.00 sec)
- 查看技术部员工的姓名
mysql> select name from employee where dep_id in(select id from department where name ="技术"); +--------+ | name | +--------+ | egon | | nvshen | +--------+ 2 rows in set (0.00 sec)
- 查看不足一人的部门名
mysql> select name from department where id not in(select dep_id from employee group by dep_id ); +--------+ | name | +--------+ | 运营 | +--------+ 1 row in set (0.00 sec
3.2 带比较运算符的子查询
