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)数据创建
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 sec3.2 带比较运算符的子查询
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