原文: http://blog.gqylpy.com/gqy/252
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目录
先准备两张表：部门表（department)、员工表（employee)
# 部门表create table department( id int primary key auto_increment, name varchar(20) not null ); # 员工表create table employee( id int primary key auto_increment, name varchar(20) not null, sex enum('male', 'female') not null default 'male', age int not null, dep_id int not null ); # 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); # 注意：department表中id=203的部门在employee中没有对应的员工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
# 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;
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
# 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# 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 | nvshen | 18 | male | 技术 |+----+---------+-----+--------+--------------+5 rows in set (0.00 sec) # 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;
外链接之左链接 left join
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 | nvshen | 技术 || 2 | alex | 人力资源 || 3 | wupeiqi | 人力资源 || 4 | yuanhao | 销售 || 6 | xiaomage | NULL |+----+----------+--------------+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
# 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 | nvshen | 技术 || NULL | NULL | 运营 |+------+---------+--------------+6 rows in set (0.00 sec)
全外链接
显示左右两个表全部记录
# 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 mysql> select * from employee left join department -> on employee.dep_id = department.id -> union all -> select * from employee right join department -> on employee.dep_id = department.id; 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 (0.00 sec) # 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
# 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mysql> select employee.name,employee.age,department.name -> from employee inner join department -> on employee.dep_id = department.id -> where age > 25;+---------+-----+--------------+| name | age | name |+---------+-----+--------------+| alex | 48 | 人力资源 || wupeiqi | 38 | 人力资源 || yuanhao | 28 | 销售 |+---------+-----+--------------+3 rows in set (0.00 sec) # 示例2 以内链接的方式查询：以age字段的升序方式显示mysql> select employee.name,employee.age,department.name -> from employee inner join department -> on employee.dep_id = department.id -> order by age asc; # 升序排序+---------+-----+--------------+| name | age | name |+---------+-----+--------------+| egon | 18 | 技术 || nvshen | 18 | 技术 || yuanhao | 28 | 销售 || wupeiqi | 38 | 人力资源 || alex | 48 | 人力资源 |+---------+-----+--------------+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
# 查询平均年龄在25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 id,name from employee where dep_id in (select id from department where name="技术"); # 查无人的部门名select name from department where id not in (select dep_id from employee);
示例2：带比较运算符的子查询
# 比较运算符：=、!=、>、>=、<、<=、<># 查询大于所有人平均年龄的员工名与年龄select name,age from employee where age > (select avg(age) from employee); # 查询大于部门内平均年龄的员工名、年龄思路： （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。 (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。 (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。 select t1.name,t1.age from employee as t1 inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age; +------+-----+| name | age |+------+-----+| alex | 48 |+------+-----+1 row in set (0.00 sec)
示例3：带exists关键字的子查询
# 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# 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 # exists为True时：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 | nvshen | male | 18 | 200 || 6 | xiaomage | female | 18 | 204 |+----+----------+--------+-----+--------+6 rows in set (0.00 sec) # exists为False时：mysql> select * from employee where exists (select id from department where id=204);Empty set (0.00 sec)
来源：https://www.cnblogs.com/bbb001/p/11367219.html