【MySQL】多表查询 -- 2019-08-07 12:53:19

原创: http://106.13.73.98/__/26/

目录

多表链接查询

笛卡尔积

内链接 inner join

外链接之左链接 left join

外链接之右链接 right join

全外链接

符合条件链接查询

子查询


先准备两张表:部门表(department)、员工表(employee)

  1. # 部门表
  2. create table department(
  3. id int primary key auto_increment,
  4. name varchar(20) not null
  5. );
  6. # 员工表
  7. create table employee(
  8. id int primary key auto_increment,
  9. name varchar(20) not null,
  10. sex enum('male', 'female') not null default 'male',
  11. age int not null,
  12. dep_id int not null
  13. );
  14. # 插入数据
  15. insert into department values
  16. (200, "技术"),
  17. (201, "人力资源"),
  18. (202, "销售"),
  19. (203, "运营")
  20. ;
  21. insert into employee(name, sex, age, dep_id) values
  22. ('egon', 'male', 18, 200),
  23. ('alex', 'female', 48, 201),
  24. ('wupeiqi', 'male', 38, 201),
  25. ('yuanhao', 'female', 28, 202),
  26. ('nvshen', 'male', 18, 200),
  27. ('xiaomage', 'female', 18, 204)
  28. ;
  29. # 注意:
  30. department表中id=203的部门在employee中没有对应的员工
  31. employee表中id=6的员工在department表中没有对应的部门

多表链接查询

  1. # 外链接语法:
  2. select 字段列表
  3. from 表1 inner|left|right join 表2
  4. on 表1.字段 = 表2.字段;

笛卡尔积:

即交叉链接
不适用任何匹配条件,生成笛卡尔积关于笛卡尔积的含义,请自行百度)

select * from employee, department;    # 语法

内链接 inner join

只链接匹配的行

  1. # 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匹配的结果
  2. # 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
  3. mysql> select
  4. -> employee.id,employee.name,employee.age,employee.sex,department.name
  5. -> from employee inner join department
  6. -> on employee.dep_id = department.id;
  7. +----+---------+-----+--------+--------------+
  8. | id | name | age | sex | name |
  9. +----+---------+-----+--------+--------------+
  10. | 1 | egon | 18 | male | 技术 |
  11. | 2 | alex | 48 | female | 人力资源 |
  12. | 3 | wupeiqi | 38 | male | 人力资源 |
  13. | 4 | yuanhao | 28 | female | 销售 |
  14. | 5 | nvshen | 18 | male | 技术 |
  15. +----+---------+-----+--------+--------------+
  16. 5 rows in set (0.00 sec)
  17. # 上述sql等同于:
  18. mysql> select
  19. -> employee.id,employee.name,employee.age,employee.sex,department.name
  20. -> from employee,department
  21. -> where employee.dep_id=department.id;

外链接之左链接 left join

优先显示左表全部记录

  1. # 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
  2. # 本质就是:在内连接的基础上增加左边有,右边没有的结果
  3. mysql> select
  4. -> employee.id,employee.name,
  5. -> department.name as depart_name
  6. -> from employee left join department
  7. -> on employee.dep_id = department.id;
  8. +----+----------+--------------+
  9. | id | name | depart_name |
  10. +----+----------+--------------+
  11. | 1 | egon | 技术 |
  12. | 5 | nvshen | 技术 |
  13. | 2 | alex | 人力资源 |
  14. | 3 | wupeiqi | 人力资源 |
  15. | 4 | yuanhao | 销售 |
  16. | 6 | xiaomage | NULL |
  17. +----+----------+--------------+
  18. 6 rows in set (0.00 sec)

外链接之右链接 right join

优先显示右表全部记录

  1. # 以右表为准,即找出所有部门信息,包括没有员工的部门
  2. # 本质就是:在内连接的基础上增加右边有,左边没有的结果
  3. mysql> select
  4. -> employee.id,employee.name,
  5. -> department.name as depart_name
  6. -> from employee right join department
  7. -> on employee.dep_id = department.id;
  8. +------+---------+--------------+
  9. | id | name | depart_name |
  10. +------+---------+--------------+
  11. | 1 | egon | 技术 |
  12. | 2 | alex | 人力资源 |
  13. | 3 | wupeiqi | 人力资源 |
  14. | 4 | yuanhao | 销售 |
  15. | 5 | nvshen | 技术 |
  16. | NULL | NULL | 运营 |
  17. +------+---------+--------------+
  18. 6 rows in set (0.00 sec)

全外链接

显示左右两个表全部记录

  1. # 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
  2. # 注意:mysql不支持全外连接 full JOIN
  3. # 强调:mysql可以使用此种方式间接实现全外连接
  4. mysql> select * from employee left join department
  5. -> on employee.dep_id = department.id
  6. -> union all
  7. -> select * from employee right join department
  8. -> on employee.dep_id = department.id;
  9. mysql> select * from employee left join department
  10. -> on employee.dep_id = department.id
  11. -> union
  12. -> select * from employee right join department
  13. -> on employee.dep_id = department.id;
  14. +------+----------+--------+------+--------+------+--------------+
  15. | id | name | sex | age | dep_id | id | name |
  16. +------+----------+--------+------+--------+------+--------------+
  17. | 1 | egon | male | 18 | 200 | 200 | 技术 |
  18. | 5 | nvshen | male | 18 | 200 | 200 | 技术 |
  19. | 2 | alex | female | 48 | 201 | 201 | 人力资源 |
  20. | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
  21. | 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
  22. | 6 | xiaomage | female | 18 | 204 | NULL | NULL |
  23. | NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
  24. +------+----------+--------+------+--------+------+--------------+
  25. 7 rows in set (0.00 sec)
  26. # 注意 union与union all的区别:union会去掉相同的纪录

符合条件链接查询

  1. # 示例1 以内链接的方式查询:找出年龄大于25岁的员工以及员工所在的部门
  2. mysql> select employee.name,employee.age,department.name
  3. -> from employee inner join department
  4. -> on employee.dep_id = department.id
  5. -> where age > 25;
  6. +---------+-----+--------------+
  7. | name | age | name |
  8. +---------+-----+--------------+
  9. | alex | 48 | 人力资源 |
  10. | wupeiqi | 38 | 人力资源 |
  11. | yuanhao | 28 | 销售 |
  12. +---------+-----+--------------+
  13. 3 rows in set (0.00 sec)
  14. # 示例2 以内链接的方式查询:以age字段的升序方式显示
  15. mysql> select employee.name,employee.age,department.name
  16. -> from employee inner join department
  17. -> on employee.dep_id = department.id
  18. -> order by age asc; # 升序排序
  19. +---------+-----+--------------+
  20. | name | age | name |
  21. +---------+-----+--------------+
  22. | egon | 18 | 技术 |
  23. | nvshen | 18 | 技术 |
  24. | yuanhao | 28 | 销售 |
  25. | wupeiqi | 38 | 人力资源 |
  26. | alex | 48 | 人力资源 |
  27. +---------+-----+--------------+
  28. 5 rows in set (0.00 sec)

子查询

1. 子查询是将一个查询语句嵌套在另一个查询语句中.
2. 内层查询语句的查询结果,可以为外层查询语句提供查询条件.
3. 子查询中可以包含:in、not in、any、all、exists、not exists 等关键字.
4. 还可以包含比较运算符:=、 !=、>、< 等.

示例1:带in关键字的子查询

  1. # 查询平均年龄在25以上的部门名
  2. select id,name from department
  3. where id in
  4. (select dep_id from employee group by dep_id having avg(age) > 25);
  5. # 查看技术部员工姓名
  6. select id,name from employee
  7. where dep_id in
  8. (select id from department where name="技术");
  9. # 查无人的部门名
  10. select name from department
  11. where id not in
  12. (select dep_id from employee);

示例2:带比较运算符的子查询

  1. # 比较运算符:=、!=、>、>=、<、<=、<>
  2. # 查询大于所有人平均年龄的员工名与年龄
  3. select name,age from employee
  4. where age > (select avg(age) from employee);
  5. # 查询大于部门内平均年龄的员工名、年龄
  6. 思路:
  7. (1)先对员工表(employee)中的人员分组(group by),查询出dep_id以及平均年龄。
  8. (2)将查出的结果作为临时表,再根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
  9. (3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。
  10. select t1.name,t1.age from employee as t1
  11. inner join
  12. (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
  13. on t1.dep_id = t2.dep_id
  14. where t1.age > t2.avg_age;
  15. +------+-----+
  16. | name | age |
  17. +------+-----+
  18. | alex | 48 |
  19. +------+-----+
  20. 1 row in set (0.00 sec)

示例3:带exists关键字的子查询

  1. # exists关键字表示存在。在使用exists关键字时,内层查询语句不返回查询记录。而是返回一个真假值:True 或 False
  2. # 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
  3. # exists为True时:
  4. mysql> select * from employee where exists (select id from department where id=200);
  5. +----+----------+--------+-----+--------+
  6. | id | name | sex | age | dep_id |
  7. +----+----------+--------+-----+--------+
  8. | 1 | egon | male | 18 | 200 |
  9. | 2 | alex | female | 48 | 201 |
  10. | 3 | wupeiqi | male | 38 | 201 |
  11. | 4 | yuanhao | female | 28 | 202 |
  12. | 5 | nvshen | male | 18 | 200 |
  13. | 6 | xiaomage | female | 18 | 204 |
  14. +----+----------+--------+-----+--------+
  15. 6 rows in set (0.00 sec)
  16. # exists为False时:
  17. mysql> select * from employee where exists (select id from department where id=204);
  18. Empty set (0.00 sec)

 

 



 

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