Question

mysql中分组如何去重

A表(字段1,字段2)(1,1),(2,2)(3,1)
B表(字段a,字段b)(1,2),(1,1),(1,3),(2,1),(3,2)
其中字段1同字段b相当于主外键关联
根据字段2统计字段a的数量,去掉字段2中数量>=2的字段a

Answer 1

order by cont ->order by count(*) select count(*) as cont,ctt_id from tmp GROUP BY ctt_id order by count(*) desc limit 0,20

追问

没看懂....这句哪里去重了select count(*) as cont,ctt_id from tmp GROUP BY ctt_id,基础比较差,麻烦大神详细告知下