如何在1个表中插入相同的数据并将其限制为10

Question

I am working in a "Recently view by user" script. I have 3 tables, tbl_user, tbl_articles and tbl_recently_viewed. My tbl_recently_viewed has 3 fields, user_id(from tbl_user),article_id(from tbl_articles) and date, and i made 2 primary keys, user_id and article_id. My problem is i want to limit a certain user_id to be inserted in tbl_recently_viewed by 10. Is there a way i can do this?

Thanks.

Answer 1

I think that you need a trigger to do what you want to do. If you are only inserting rows, then you can use an insert trigger to remove the oldest.

The following is intended just as an example:

CREATE TRIGGER t_keep10 after INSERT ON t
  FOR EACH ROW BEGIN
     if (10 > (select count(*) from t where t.user_id = new.user_id)) then
         delete from t where t.article_id = (select a from (select article_id as a from t t2 where t2.user_id = t.user_id order by date desc limit 1))
     end if;
  END;
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Answer 2

To delete the oldest item for a user from tbl_recently_viewed :

DELETE v.* FROM tbl_recently_viewed v
JOIN (SELECT MIN(date) mindate
      FROM tbl_recently_viewed
      WHERE user_id = ?) r
ON v.date = r.mindate
WHERE user_id = ?

Answer 3

You could delete rows after 10 with this query:

delete from tbl_recently_viewed where user_id=1 and date <= (select max(date) from (select date from tbl_recently_viewed where user_id=1 order by date desc limit 10,999) as tmp);

or just delete oldest row with query:

delete from tbl_recently_viewed where user_id=1 order by date limit 1;