一：多表连接查询
- 外连接语法：
- select 字段列表
from 表1 inner|left|right join 表2
on 表1.字段 = 表2.字段；
（1）直接交叉连接：生成笛卡儿积，不适用于任何匹配条件
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 | 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 |
(2) 内连接：只连接匹配的行
mysql> select * from employee as A inner join department as B on A.dep_id = B.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 (0.09 sec)
(3) 左连接：优先显示左表全部记录
mysql> select * from employee as A left join department as B on A.dep_id = B.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 (0.57 sec)
（4）右连接：优先显示右表全部记录
mysql> select * from employee as A right join department as B on A.dep_id = B.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)
（5）全外连接：显示左右两个表全部记录
mysql> select * from employee as A left join department as B on A.dep_id = B.id -> union select * from employee as A right join department as B on A.dep_id = B.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)
二：符合条件连接查询
- 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 (0.04 sec)
三：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，all，exists 和not exists 等关键字
4.还可以包含比较运算符：= ，!=，> , < 等
（1）in 关键字
#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);
（2）比较运算符
#查询大于所有人平均年龄的员工名与年龄 mysql> 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,再对根据临时表的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。 mysql> 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;