标签：lse style color 一个 连接 pen one sts nio
一.多表连接查询
1.交叉连接:不适用任何匹配条件,生成笛卡儿积
select * from 表1,表2;
2.内连接:只连接匹配的行
select * from 表1,表2 where 表1.字段 = 表2.字段;
select * from 表1 inner join 表2 on 表1.字段 = 表2.字段;
3.左连接:优先显示左表全部记录
select * from 表1 left join 表2 on 表1.字段 = 表2.字段;
4.右连接:优先显示右表全部记录
select * from 表1 right join 表2 on 表1.字段 = 表2.字段;
5.全外连接:显示两个表全部记录,union all 显示两个表拼接的有重复的,union可以去重.
select * from 表1 left join 表2 on 表1.字段 = 表2.字段
union
select * from 表1 right join 表2 on 表1.字段 = 表2.字段;
二.子查询
子查询是将一个查询语句嵌套在另一个查询语句中,子查询中的关键字有in,not in , all ,any ,exists,not exists,还有运算符 = ,!= ,< ,>
1.in关键字子查询
#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‘);
2.exists关键字子查询
exists关键字表示存在,如果exists后面的内层查询存在就查询外层,否则不进行查询.
#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)
3.比较运算符子查询
#比较运算符：=、!=、>、>=、<、<=、<> #查询大于所有人平均年龄的员工名与年龄 mysql> select name,age from emp where age > (select avg(age) from emp); +---------+------+ | name | age | +---------+------+ | alex | 48 | | wupeiqi | 38 | +---------+------+ rows in set (0.00 sec) #查询大于部门内平均年龄的员工名、年龄 select t1.name,t1.age from emp t1 inner join (select dep_id,avg(age) avg_age from emp group by dep_id) t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age;
标签：lse style color 一个 连接 pen one sts nio
原文地址：https://www.cnblogs.com/q767498226/p/10296585.html
