sql 分组的两种数量查询
1、以teacher_id进行分组并且查出每个分组里有多少成员
//如果有where条件就在Dbname后加入where条件即可
SELECT COUNT(*) AS every_group_nums FROM DBname GROUP BY teacher_id;
2、以teacher_id进行分组，然后查出一共有多少个分组
//必须有（AS a）//a是随便起的
SELECT COUNT(*) AS all_groups FROM (SELECT teacher_id FROM Dbname GROUP BY teacher_id) AS a;