删除表中多余的重复记录，重复记录是根据单个字段（id）来判断，只留有rowid最小的记录 

//删除用户 根据用户名，并且不包括最小ID
delete from users where username in (
         --根据用户名称排序，大于1
	select username from users group by username having count(username) > 1
 
 ) and Id not in (
        --根据用户名排序，获取最小ID
	select min(Id) from users group by username having count(username) > 1
);

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