### group by ~~~ select gender from students group by gender; ~~~ ### group by + group_concat() ~~~ 分组之后，根据分组结果，使用group_concat()来放置每一组的某字段的值的集合 select gender,group_concat(name) from students group by gender; ~~~ ### group by + having 1. having 条件表达式：用来分组查询后指定一些条件来输出查询结果 2. having作用和where一样，但having只能用于group by ~~~ select gender,count(*) from students group by gender having count(*)>2; ~~~ ### group by + with rollup with rollup的作用是：在最后新增一行，来记录当前列里所有记录的总和 ~~~ select gender,count(*) from students group by gender with rollup; ~~~