1 去重
1.1 查询
1.1.1 存在部分字段相同的纪录，即有唯一键主键ID
最常见情况如果是这种情况的话用distinct是过滤不了的，这就要用到主键id的唯一性特点及group by分组
select * from table where id in (select max(id) from table group by [去除重复的字段名列表,....])
1.1.2 存在两条完全相同的记录用关键字distinct就可以去掉
select distinct id(某一列) from table(表名) where (条件)
1.1.3 查找表中不含重复的数据，根据单个字段（id）来判断
select * from table where id in (select id from table group by id having count (id) >1)
1.1.4 查找表中重复的数据，根据单个字段（id）来判断
select * from table where id not in (select id from table group by id having count (id) >1)
1.1.5 查询全部的重复信息
select * from people where id not in (select min(id) from people group by name,sex HAVING COUNT(*) < 2)
1.1.6 查询全部的重复信息
select * from table where id not in (select MIN(id) from table group by name,sex)
1.1.7 删除多余重复的信息，只保留最小ID
delete from table where id not in(select MIN(id) from table group by name,sex)
