Python全栈（三）数据库优化之4.数据库查询（分组、排序、分页、连接查询、子查询）

一、分组与分组后的筛选

1.分组

关键字：group by
将查询结果按照1个或多个字段进行分组，字段值相同的为一组；
可用于单个字段分组，也可用于多个字段分组
语法：select ... from 表 group by 组别;

select name from students group by gender;

打印

ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'demo.students.NAME' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

报错，并且没有实际意义，即select后接的是能区分这个组的字段，一般是聚合函数。

select gender from students group by gender;

打