Question

我不太确定如何编写此查询，可以在一个查询中完成。情况就是这样：

我需要选择一个标签名称列表，并为每个标签获取最近标记的相册信息。意思是如果用户创建一个名为“Pamela Anderson”的专辑并将该专辑标记为“Blondes”，则该专辑现在是Blondes最近标记的专辑。可能有100张专辑标记为“Blondes”，但只应选择最近的一张专辑，由objectTagCreateDate确定。

预期数据如下所示：

    tagId, tagName,   objectTagCreateDate, albumName,       albumPath
   --------------------------------------------------------------------
    1,     Blondes,   2011-08-23,          Pamela Anderson,   7345
    2,     Celebs,    2011-08-20,          Kate Winslet,    2, 5567
    3,     Musicians, 2011-08-22,          Alicia Keys,       6678

等

表格如下：

Tag
 - tagId
 - tagName

ObjectTag
 - objectType
 - objectId
 - tagId
 - objectTagCreateDate

Album
 - albumId
 - albumName
 - albumPath

联接发生在：

tag.tagId = objectTag.tagId AND objectTag.objectType = 3 and objectTag.objectId = album.albumId

这可以在一个查询中完成，如果是这样的话怎么办？感谢

Answer 1

SELECT t.tagId, t.tagName, o.objectTagCreateDate, a.albumName, a.albumPath
FROM ObjectTag AS o
INNER JOIN (
            SELECT tagId, MAX(objectTagCreateDate) As MaxDate
            FROM ObjectTag
            WHERE ObjectTag.objectType = 3
            GROUP BY tagId
            ) AS t1
ON t1.tagId = o.tagId AND t1.MaxDate = o.objectTagCreateDate
INNER JOIN Tag AS t ON t1.tagId = t.tagId
INNER JOIN Album AS a ON o.objectID = a.albumId

内部选择获取每个tagId MaxDate并与ObjectTag结合以获得相应的o.objectID。其余的只是简单的连接。

Answer 2

这样的事情应该这样做，但我自己没有测试过它;

SELECT T.tagId, T.tagName, MAX(OT.objectTagCreateDate) AS LatestObjectTagCreateDate, A.albumName, A.albumPath
FROM ObjectTag OT
JOIN Tag T ON (T.tagId = OT.tagId)
JOIN Album A ON (A.albumId = OT.objectId AND OT.objectType = 3)
GROUP BY T.tagId
ORDER BY T.tagId

帮助MySql查询

2 个答案: