插入 并且 所有 方式 color dep join 查看表
本节主题
多表连接查询
复合条件连接查询
子查询
准备表
company.employee
company.department
- #建表
- 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),
- (‘liwenzhou‘,‘male‘,18,200),
- (‘jingliyang‘,‘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 | egon | male | 18 | 200 |
- | 2 | alex | female | 48 | 201 |
- | 3 | wupeiqi | male | 38 | 201 |
- | 4 | yuanhao | female | 28 | 202 |
- | 5 | liwenzhou | male | 18 | 200 |
- | 6 | jingliyang | female | 18 | 204 |
- +----+------------+--------+------+--------+
- #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 | 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 | liwenzhou | male | 18 | 200 | 200 | 技术 |
- | 5 | liwenzhou | male | 18 | 200 | 201 | 人力资源 |
- | 5 | liwenzhou | male | 18 | 200 | 202 | 销售 |
- | 5 | liwenzhou | male | 18 | 200 | 203 | 运营 |
- | 6 | jingliyang | female | 18 | 204 | 200 | 技术 |
- | 6 | jingliyang | female | 18 | 204 | 201 | 人力资源 |
- | 6 | jingliyang | female | 18 | 204 | 202 | 销售 |
- | 6 | jingliyang | 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 | egon | 18 | male | 技术 |
- | 2 | alex | 48 | female | 人力资源 |
- | 3 | wupeiqi | 38 | male | 人力资源 |
- | 4 | yuanhao | 28 | female | 销售 |
- | 5 | liwenzhou | 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;
3 外链接之左连接:优先显示左表全部记录
- #以左表为准,即找出所有员工信息,当然包括没有部门的员工
- #本质就是:在内连接的基础上增加左边有右边没有的结果
- mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
- +----+------------+--------------+
- | id | name | depart_name |
- +----+------------+--------------+
- | 1 | egon | 技术 |
- | 5 | liwenzhou | 技术 |
- | 2 | alex | 人力资源 |
- | 3 | wupeiqi | 人力资源 |
- | 4 | yuanhao | 销售 |
- | 6 | jingliyang | NULL |
- +----+------------+--------------+
4 外链接之右连接:优先显示右表全部记录
- #以右表为准,即找出所有部门信息,包括没有员工的部门
- #本质就是:在内连接的基础上增加右边有左边没有的结果
- mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
- +------+-----------+--------------+
- | id | name | depart_name |
- +------+-----------+--------------+
- | 1 | egon | 技术 |
- | 2 | alex | 人力资源 |
- | 3 | wupeiqi | 人力资源 |
- | 4 | yuanhao | 销售 |
- | 5 | liwenzhou | 技术 |
- | NULL | NULL | 运营 |
- +------+-----------+--------------+
5 全外连接:显示左右两个表全部记录
- 全外连接:在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
- #注意:mysql不支持全外连接 full JOIN
- #强调:mysql可以使用此种方式间接实现全外连接
- 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
- ;
- #查看结果
- +------+------------+--------+------+--------+------+--------------+
- | id | name | sex | age | dep_id | id | name |
- +------+------------+--------+------+--------+------+--------------+
- | 1 | egon | male | 18 | 200 | 200 | 技术 |
- | 5 | liwenzhou | 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 | jingliyang | female | 18 | 204 | NULL | NULL |
- | NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
- +------+------------+--------+------+--------+------+--------------+
- #注意 union与union all的区别:union会去掉相同的纪录
- #示例1:以内连接的方式查询employee和department表,并且employee表中的age字段值必须大于25,
- 即找出公司所有部门中年龄大于25岁的员工select employee.name,
- employee.age from employee,
- department where employee.dep_id = department.id and age > 25;
- #示例2:以内连接的方式查询employee和department表,并且以age字段的升序方式显示select employee.id,
- employee.name,
- employee.age,
- department.name from employee,
- department where employee.dep_id = department.id and age > 25 order by age asc;
- #比较运算符:=、!=、>、>=、<、<=、<>
- #查询平均年龄在25岁以上的部门名
- select id,name from department
- where id in
- (select dep_id from employee group by dep_id having avg(age) > 25);
- #查看技术部员工姓名
- select name from employee
- where dep_id in
- (select id from department where name=‘技术‘);
- #查看不足1人的部门名
- select name from department
- where id in
- (select dep_id from employee group by dep_id having count(id) <=1);
3 带EXISTS关键字的子查询
EXISTS关字键字表示存在。在使用EXISTS关键字时,内层查询语句不返回查询的记录。
而是返回一个真假值。True或False
当返回True时,外层查询语句将进行查询;当返回值为False时,外层查询语句不进行查询
- #department表中存在dept_id=203,Ture
- mysql> select * from employee
- -> where exists
- -> (select id from department where id=200);
- +----+------------+--------+------+--------+
- | 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 | liwenzhou | male | 18 | 200 |
- | 6 | jingliyang | female | 18 | 204 |
- +----+------------+--------+------+--------+
- #department表中存在dept_id=205,False
- mysql> select * from employee
- -> where exists
- -> (select id from department where id=204);
- Empty set (0.00 sec)
mysql数据库从删库到跑路之mysql多表查询
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