5.更多查询-条件查询、聚合查询
条件查询
比较:大于>,等于=,小于<,不等<>
通配:select*frm where customer_name like '%or%'
范围限定:where pricer between 10 and 20;
子集限定:where country in ('red','green','yellow','black')
逻辑关系:and or not
注意⚠️子集限定还可以在in(select*from)在in中添加select语句
聚合查询
基本语法:group by 字段 having 条件
常用函数:COUNT ,MAX ,MIN,SUM,AVG
例子:1)select count(gender),gender from employees group by gneder;
2)select count(gender),gender from employees group by gneder having gender='F';
分组聚合查询的优点:
到底有多少组取决于这个数据库字段里有多少值,不会漏掉字段
6.多表查询