Question

我一直在尝试从多个表中选择数据，而我按照我用左连接加入的附加表中的id计数排序。一切都很好，除了计数增加600something而不是每行一个。

我尝试了，因为我在类似的问题中看到将不同的内容放在count语句中，但只有冻结数据库的结果。

表：

CREATE TABLE IF NOT EXISTS `places` (
  `PlaceId` int(10) NOT NULL AUTO_INCREMENT,
  `Name` varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  `AreaId` int(10) NOT NULL DEFAULT '1',
  PRIMARY KEY (`PlaceId`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=0;

CREATE TABLE IF NOT EXISTS `places_descriptions` (
  `DescId` int(45) NOT NULL AUTO_INCREMENT,
  `PlaceId` int(10) NOT NULL,
  `Description` varchar(1024) COLLATE utf8_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`DescId`),
  KEY `PlaceId` (`PlaceId`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=0;

CREATE TABLE IF NOT EXISTS `places_hits` (
  `HitId` int(45) NOT NULL AUTO_INCREMENT,
  `PlaceId` int(45) NOT NULL,
  PRIMARY KEY (`HitId`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=0;

查询：

SELECT 
    p.PlaceId, 
    p.Name, 
    pd.Description,
    COUNT(ph.HitId) AS numHits
FROM 
    places_descriptions AS pd, 
    places AS p 
    LEFT JOIN places_hits AS ph 
    ON (p.PlaceId = ph.PlaceId) 
WHERE 
    (p.PlaceId = pd.PlaceId) AND 
    (p.AreaId = 1) 
GROUP BY 
    p.PlaceId, 
    p.Name, 
    pd.Description 
ORDER BY 
    numHits DESC, 
    p.PlaceId 
LIMIT 0, 10

有什么想法吗？谢谢！

Answer 1

乍一看，您的问题可能是您正在交叉加入pd和p，因为没有ON子句。 placeId = placeId在where中通过过滤掉连接不准确的行来掩盖此问题，但最好将该条件置于ON中。过滤前实际存在的大量行可能会丢掉计数，但我不确定。

试试这个，开始：

SELECT 
    p.PlaceId, 
    p.Name, 
    pd.Description,
    COUNT(ph.HitId) AS numHits
FROM 
    places_descriptions AS pd
    INNER JOIN places AS p 
            ON p.PlaceId = pd.PlaceID
    LEFT JOIN places_hits AS ph 
    ON (p.PlaceId = ph.PlaceId) 
WHERE 
    (p.AreaId = 1) 
GROUP BY 
    p.PlaceId, 
    p.Name, 
    pd.Description 
ORDER BY 
    numHits DESC, 
    p.PlaceId

MySQL从多个表中选择按左连接表的数量排序

1 个答案: