MySQL，如何摆脱重复的记录，联接？

Question

Lets imagine two tables:

username
 id, name
 1   username1
 2   username1
 3   username2

emails
 id, user, name
 1   1     assdf@sdf.hu
 2   2     afgdf@sdf.hu
 3   3     gfg@sdf.hu

the problem is there are duplicated entries. I cant simply drop them, because records are connected to them, so they get lost. (for example afgdf@sdf.hu email address). How to solve it?

Answer 1

update emails set user=1 where user=2 ，然后您只需在用户名表中删除包含id 2的行

Answer 2

update your username tabel. For every name that is duplicate (triplet, quat...), update the emails table and replace the user with the original user id.

Answer 3

SQLFiddle demo

First you should update table emails to replace user with right values to eliminate values to be deleted.

update emails set user=(select min(id) 
                        from username 
                       where name=
                      (select name from username where id=emails.user));

Now delete from username table duplicate records. We keep only records with MINIMAL id for each name :

DELETE t1.*
FROM username t1
LEFT JOIN (select min(id) mid from username group by name) t2 ON t1.ID = t2.mid
WHERE t2.mid is null;