ON子句中的MySQL未知列

Question

我有以下MySQL查询:

SELECT p.*,
    IF(COUNT(ms.PropertyID) > 0,1,0) AS Contacted,
    pm.MediaID,
    date_format(p.AvailableFrom, '%d %b %Y') AS 'AvailableFrom',
    astext(pg.Geometry) AS Geometry
FROM property p, propertygeometry pg
    JOIN shortlist sl ON sl.PropertyID = p.id AND sl.MemberID = 384216
    LEFT JOIN message ms ON ms.PropertyID = p.id AND ms.SenderID = 384216
    LEFT JOIN property_media pm ON pm.PropertyID = p.id AND pm.IsPrimary = 1
WHERE p.paused = 0
    AND p.PropertyGeometryID = pg.id
GROUP BY p.id

我收到这个错误:

#1054 - 'on子句'中的未知列'p.id'

据我所知,查询看起来正确,任何想法可能是错的？

Answer 1

不要混用ANSI-89样式和ANSI-92样式的连接.它们具有不同的优先级,可能导致混淆错误,这就是这里发生的事情.您的查询解释如下:

FROM property p, (
    propertygeometry pg
    JOIN shortlist sl ON sl.PropertyID = p.id AND sl.MemberID = 384216
    ...
)

在上面,在考虑逗号样式连接之前,首先计算使用JOIN关键字的连接.此时该表p尚未声明.

从MySQL手册:

但是,逗号运算符的优先级小于INNER JOIN,CROSS JOIN,LEFT JOIN等.如果在存在连接条件时将逗号连接与其他连接类型混合,则可能会出现'on子句'中未知列'col_name'形式的错误.有关处理此问题的信息将在本节后面给出.

我建议始终使用ANSI-92样式连接,即使用JOIN关键字:

SELECT p.*,
    IF(COUNT(ms.PropertyID) > 0,1,0) AS Contacted,
    pm.MediaID,
    date_format(p.AvailableFrom, '%d %b %Y') AS 'AvailableFrom',
    astext(pg.Geometry) AS Geometry
FROM property p
    JOIN propertygeometry pg ON p.PropertyGeometryID = pg.id
    JOIN shortlist sl ON sl.PropertyID = p.id AND sl.MemberID = 384216
    LEFT JOIN message ms ON ms.PropertyID = p.id AND ms.SenderID = 384216
    LEFT JOIN property_media pm ON pm.PropertyID = p.id AND pm.IsPrimary = 1
WHERE p.paused = 0
GROUP BY p.id

有关:

• 为什么SQL ANSI-92标准不比ANSI-89更好地采用？

Answer 2

如前所述,通过逗号运算符使用连接存在优先级问题,其中将执行LEFT JOIN,因此此时不存在对表别名的引用.虽然您可以通过该语句隐式告诉MySQL使用JOIN,但您也可以告诉MySQL先评估逗号连接表,然后执行left join:

SELECT p.*,
IF(COUNT(ms.PropertyID) > 0,1,0) AS Contacted,
pm.MediaID,
date_format(p.AvailableFrom, '%d %b %Y') AS 'AvailableFrom',
astext(pg.Geometry) AS Geometry
FROM (property p, propertygeometry pg)
JOIN shortlist sl ON sl.PropertyID = p.id AND sl.MemberID = 384216
LEFT JOIN message ms ON ms.PropertyID = p.id AND ms.SenderID = 384216
LEFT JOIN property_media pm ON pm.PropertyID = p.id AND pm.IsPrimary = 1
WHERE p.paused = 0
AND p.PropertyGeometryID = pg.id
GROUP BY p.id

请注意,逗号分隔的表包含在括号()中.表别名和列现在可供您的其他JOIN使用.