mysql的分组查询、分组统计

查询主题下的二级回复和三级回复列表

SELECT

a.topic_id,count(*) num
FROM

diz_reply AS a where a.parent_id<>'0' GROUP BY a.topic_id

查询一个班的男生数和女生数：

SELECT   grade, COUNT (CASE WHEN sex = 1 THEN 1      /*sex 1为男生，2位女生*/
                       ELSE NULL
                       END) 男生数,
                COUNT (CASE WHEN sex = 2 THEN 1
                       ELSE NULL
                       END) 女生数
    FROM students GROUP BY grade;