Mysql 一对多关系

Question

I have two tables:

TABLE items with IDItem field TABLE comments with IDcomment, IDItem, datePublished and comment fields.

If I want to list last 10 comments it is no problem, I just sort the 'comments' table. The problem is when I want to list last ten comments on the individual items, that means items are not duplicated.

Any best way to achieve this in regards to using indexes? If I order by 'comments' and group by IDItem I don't get the last comment out on each item as the group seems to order randomly:-(

I found solution to bring 'lastDate' to the 'items' table, so I can sort by the items and I will have the correct sort order, but when I join to the comments table I get 10 rows of the same item id if it had 10 comments:-(

How is the proper way to join one to many so I get only one item from the left table with one item on the right table?

I am not sure if I was very clear.

Answer 1

It sounds like you're trying to get the 10 items returned that have the most recent 10 comments, with one comment per item correct?

If so, try this:

SELECT * FROM Items I
JOIN
(SELECT TOP 10 * FROM Comments C2 WHERE DatePublished=
       (SELECT MAX(DatePublished) FROM Comments C3 WHERE C2.IDItem=C3.IDItem)
       ORDER BY DatePublished DESC) C1
ON I.IDItem=C1.IDItem

Edited: Removed extra SELECT and added limit of 10 comments returned

Answer 2

DECLARE @item TABLE
(
  IDItem int
)
DECLARE @comment TABLE
(
  IDComment int,
  DatePublished date,
  IDItem int,
  Comment varchar(100)
)

INSERT INTO @item (IDItem) VALUES (1);
INSERT INTO @item (IDItem) VALUES (2);
INSERT INTO @item (IDItem) VALUES (3);
INSERT INTO @item (IDItem) VALUES (4);

INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (1,'2011-01-01', 1, 'test1');
INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (2,'2011-01-02', 1, 'test2');
INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (3,'2011-01-01', 2, 'test3');
INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (4,'2011-01-03', 2, 'test4');
INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (5,'2011-01-02', 3, 'test5');
INSERT INTO @comment (IDComment, DatePublished, IDItem, Comment) VALUES (6,'2011-01-05', 3, 'test6');

SELECT i.IDItem, (SELECT TOP 1 c.Comment FROM @comment c WHERE c.IDItem = i.IDItem ORDER BY c.DatePublished) FROM @item i

RETURNS

   1    test1
   2    test3
   3    test5
   4    NULL

If it's what are you looking for, just mysql this code. Replace TOP 1 with LIMIT 1 etc.

Answer 3

(Updated) What about this?

SELECT IDcomment, IDitem from COMMENTS where IDitem in (SELECT DISTINCT(IDitem) FROM comments);