Why operator LIMIT does not work correctly? - mysql

Question

I wrote the next query SQL for getting last comments with re-comments:
SELECT c.*, ar.ArticleName, ar.idArticle, du.DetailToUsersName, du.DetailToUsersPhoto, COUNT(c.idCommentToArticle) AS CNT, CASE WHEN d.Count IS NULL THEN 0 ELSE d.Count END AS CountLikes
FROM commenttoarticle c
INNER JOIN (SELECT CommentToArticlePID FROM commenttoarticle
GROUP BY CommentToArticlePID
ORDER BY CommentToArticlePID LIMIT 3) AS articleComments
USING (CommentToArticlePID)
LEFT JOIN article ar ON c.CommentToArticleIdArticle = ar.idArticle
LEFT JOIN detailtousers du ON du.idDetailToUsers = c.CommentToArticleIdUser
LEFT JOIN `likes` d ON (d.IdNote = c.idCommentToArticle AND d.LikeType = 6)
WHERE c.CommentToArticleIdArticle = 11
GROUP BY c.idCommentToArticle
So, why operator LIMIT 3 in sub-query select does not work? Now this query shows all rows from table commenttoarticle
I seem that need do somethink like as:
SELECT...
FROM (select * from commenttoarticle commenttoarticle c INNER JOIN
(SELECT distinct(CommentToArticlePID)
FROM commenttoarticle b
ORDER BY CommentToArticlePID
LIMIT 2) AS commenttoarticle USING (CommentToArticlePID)) as c
LEFT JOIN article ar ON c.CommentToArticleIdArticle = ar.idArticle...
Dump table commenttoarticle:
CREATE TABLE IF NOT EXISTS `commenttoarticle` (
`idCommentToArticle` int(11) NOT NULL AUTO_INCREMENT,
`CommentToArticleTime` int(11) NOT NULL,
`CommentToArticleIdArticle` int(11) NOT NULL,
`CommentToArticleComment` text NOT NULL,
`CommentToArticleIdUser` int(11) NOT NULL,
`CommentToArticlePID` int(11) NOT NULL,
PRIMARY KEY (`idCommentToArticle`),
UNIQUE KEY `idCommentToArticle_UNIQUE` (`idCommentToArticle`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=59 ;
INSERT INTO `commenttoarticle` (`idCommentToArticle`, `CommentToArticleTime`, `CommentToArticleIdArticle`, `CommentToArticleComment`, `CommentToArticleIdUser`, `CommentToArticlePID`) VALUES
(29, 0, 11, 'продажам?\nИнтересует не мега-звезда, а именно предметный, руками умеющий продавать сам и помогающий выстраивать это бизнесам.', 459, 0),
(30, 0, 11, '2', 459, 0),
(31, 0, 11, '3', 459, 0),
(36, 0, 11, '3.1', 459, 31),
(37, 1413822798, 11, 'also facing that prob. on the plteform of win 7', 459, 29),
(38, 0, 11, ' here i dont have internet connection.. #Samint Sinha thanks ill check it out maybe tomorrow.', 459, 29),
(39, 0, 11, ' Select max id and you will have dhe last row returned', 459, 29),
(32, 0, 11, '4', 459, 0),
(44, 1414354324, 11, 'How to do', 456, 29),
(45, 1414354469, 11, 'sfsfsf', 456, 29),
(46, 1414354708, 11, 'dddd', 456, 29),
(47, 1414357761, 11, 'sfsfs', 456, 0),
(57, 1414370833, 39, 'kkkppppppp', 456, 0),
(49, 1414358233, 11, 'VSF\nSFSF', 456, 0),
(50, 1414359589, 11, 'How to do', 456, 0),
(51, 1414359660, 11, 'sfsfsdf', 456, 0),
(52, 1414361057, 11, 'SDFSF', 456, 0),
(53, 1414364023, 11, 'dsfdsjfsifmsi', 456, 0),
(54, 1414364031, 11, 'sdfdskjfnskf', 456, 52),
(55, 1414364034, 11, 'sdfdskjfnskf', 456, 52),
(56, 1414364044, 11, 'fndsdfnsofosfi', 456, 52),
(58, 1414370841, 39, 'dfgdfgdgdgdgdgdfgdgdfg', 456, 0);
Dump table article:
CREATE TABLE IF NOT EXISTS `article` (
`idArticle` int(11) NOT NULL AUTO_INCREMENT,
`ArticleName` varchar(255) NOT NULL,
`ArticleTime` int(11) NOT NULL,
`ArticleDescription` varchar(500) NOT NULL,
`ArticleText` text NOT NULL,
`ArticleToUserID` int(11) DEFAULT NULL,
`ArticleCategory` int(11) NOT NULL,
`ArticleView` int(11) NOT NULL,
`ArticleCountry` int(11) NOT NULL,
`ArticlePhoto` varchar(150) NOT NULL,
`ArticleCity` int(11) NOT NULL,
PRIMARY KEY (`idArticle`),
UNIQUE KEY `idArticle_UNIQUE` (`idArticle`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=50 ;

Answer 1

By constructing a self join on comments yuo're not limiting but multiply the result.
What you need is a single subquery on comments table
SELECT c.*, ar.ArticleName,
ar.idArticle, du.DetailToUsersName,
du.DetailToUsersPhoto, COALECSE(SUM(d.count), 0)
FROM
(SELECT *
FROM
commenttoarticle
WHERE
CommentToArticleIdArticle = 11
ORDER BY
CommentToArticlePID, idCommentToArticle DESC
LIMIT 3
) c
LEFT JOIN
article ar
ON c.CommentToArticleIdArticle = ar.idArticle
LEFT JOIN
detailtousers du
ON du.idDetailToUsers = c.CommentToArticleIdUser
LEFT JOIN
likes d
ON (d.IdNote = c.idCommentToArticle AND d.LikeType = 6)
GROUP BY
c.idCommentToArticle
I have made the assumption that a comment can have multiple entries in the likes table, otherwise the subquery and the group by is not necesarry.

Answer 2

The USING(column_list) clause names a list of columns that must exist in both tables.
reference: http://dev.mysql.com/doc/refman/5.0/en/join.html
CommentToArticlePID is not in your first table.
Also, what is the point of joining a single column? Yes, if you want to limit, this might be applicable, but then why us LIMIT?