sql note分组查询

1.分组查询
按照指定字段进行分组，字段中数据相等的分为一组
group by 列名 [having 条件表达式] [with rollup]

• 列名：按照指定字段的值进行分组
• having 条件表达式：用来过滤分组后的数据
• with rollup:在所有记录的最后加上一条记录，显示select查询时聚合函数的统计和计算结果
  2.group by使用
  select gender from students group by gender;
  select name, gender from students group by name, gender;
  3.group by + group_concat()
  group_concat(字段名）：统计每个分组指定字段的信息集合，每个信息之间使用逗号进行分割。
  select gender, group_concat(name) from students group by gender;
  e.g.
  select gender, group_concat(name) from students group by gender;
  select gender, count() form students group by gender ;
  select gender, avg(age) from students group by gender;
  select gender, count() from students group by gender having count(*) >2;
  select gender, sum(age) from students group by gender;