### group by ~~~ select gender from students group by gender; ~~~ ### group by + group_concat() ~~~ 分組之后，根據分組結果，使用group_concat()來放置每一組的某字段的值的集合 select gender,group_concat(name) from students group by gender; ~~~ ### group by + having 1. having 條件表達式：用來分組查詢后指定一些條件來輸出查詢結果 2. having作用和where一樣，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，來記錄當前列里所有記錄的總和 ~~~ select gender,count(*) from students group by gender with rollup; ~~~