Question

我正在尝试运行此查询：

SELECT 
  Destaque.destaque, Noticia.id, Noticia.antetitulo, 
  Noticia.titulo, Noticia.lead, Noticia.legenda, 
  Noticia.publicacao, Seccao.descricao, Album.pasta,
  Foto.ficheiro, Foto.descricao, Cronista.nome, 
  Cronista.profissao, Cronista.ficheiro,
  AudioFile.*, AudioCollection.*, VideoFile.*, VideoCollection.*
FROM 
  nt_highlights AS Destaque
  LEFT JOIN nt_noticias  AS Noticia         ON Destaque.noticia_id = Noticia.id
  LEFT JOIN mm_fotos     AS Foto            ON Noticia.foto_id = Foto.id
  LEFT JOIN nt_temas     AS Seccao          ON Noticia.tema_id = Seccao.id
  LEFT JOIN mm_albuns    AS Album           ON Foto.album_id = Album.id
  LEFT JOIN nt_cronistas AS Cronista        ON Cronista.id = Noticia.cronista_id  
  LEFT JOIN ntNoticias_mmFiles AS Rel       ON Rel.noticia_id = Noticia.id
  LEFT JOIN mm_files     AS AudioFile       ON AudioFile.id = Rel.file_id
  LEFT JOIN mm_coleccoes AS AudioCollection ON AudioFile.coleccao_id = AudioCollection.id        
  LEFT JOIN mm_files     AS VideoFile       ON VideoFile.id = Rel.file_id
  LEFT JOIN mm_coleccoes AS VideoCollection ON VideoFile.coleccao_id = VideoCollection.id
WHERE 
  Destaque.area_id = 1
  AND Noticia.paraPublicacao = 1 
  AND Noticia.publicacao <= NOW()   
  AND (AudioFile.mimeType != '' OR AudioFile.id IS NULL)
  AND (VideoFile.mimeType = '' OR VideoFile.id IS NULL)
ORDER BY 
  Destaque.destaque

这会给我一些文章（来自nt_noticias），我们的想法是同时从Video获取Audio和mm_files文件表

当我有一篇有声音和视频的文章时，MySQL会返回4行：

声音（视频为空）
与视频（声音为空）
with all nulls
声音和视频

我如何“强制”它每篇文章只返回一行，并且与现有的视频和音频相关联？我在这里做错了什么？

Answer 1

JOIN将返回所有组合，这就是问题如果每篇文章只有一个音频和/或视频文件，那么您可能需要查看子选择。在SQL Server中，这看起来像（未经测试的代码）：

SELECT title, 
       (select TOP 1 audio from audio where audio.aid = articles.id) as Audio, 
       (select TOP 1 video from video where video.aid = articles.id) as Video
FROM articles

请注意，在大型数据集上，这可能会表现不佳，因为此示例中的子选择是针对返回到外部查询的每一行单独执行的。例如，如果您返回10,000篇文章，那么实际上将在服务器上执行总共20,001个查询。还有其他可能的答案可以解决这个问题，但是他们会更多地参与其中（我怀疑你可以用衍生的表做些什么，但目前我不能这样做。）

Answer 2

我想你想要这样的东西：

SELECT 
  Destaque.destaque, Noticia.id, Noticia.antetitulo, 
  Noticia.titulo, Noticia.lead, Noticia.legenda, 
  Noticia.publicacao, Seccao.descricao, Album.pasta,
  Foto.ficheiro, Foto.descricao, Cronista.nome, 
  Cronista.profissao, Cronista.ficheiro,
  AudioFile.*, AudioCollection.*, VideoFile.*, VideoCollection.*
FROM 
  nt_highlights AS Destaque
  LEFT JOIN nt_noticias  AS Noticia         ON Destaque.noticia_id = Noticia.id
  LEFT JOIN mm_fotos     AS Foto            ON Noticia.foto_id = Foto.id
  LEFT JOIN nt_temas     AS Seccao          ON Noticia.tema_id = Seccao.id
  LEFT JOIN mm_albuns    AS Album           ON Foto.album_id = Album.id
  LEFT JOIN nt_cronistas AS Cronista        ON Cronista.id = Noticia.cronista_id  
  LEFT JOIN ntNoticias_mmFiles AS AudioRel  ON Rel.noticia_id = Noticia.id
                                               AND AudioRel.file_id IN (
    SELECT file_id 
    FROM   ntNoticias_mmFiles 
    WHERE  noticia_id = Noticia.id AND IsAudioFile = 1 /* whatever the check is */
    LIMIT  1
  )
  LEFT JOIN mm_files     AS AudioFile       ON AudioFile.id = Rel.file_id
  LEFT JOIN mm_coleccoes AS AudioCollection ON AudioFile.coleccao_id = AudioCollection.id        
  LEFT JOIN ntNoticias_mmFiles AS VideoRel  ON VideoRel.noticia_id = Noticia.id
                                               AND VideoRel.file_id IN (
    SELECT file_id 
    FROM   ntNoticias_mmFiles 
    WHERE  noticia_id = Noticia.id AND IsVideoFile = 1  /* whatever the check is */
    LIMIT  1
  )
  LEFT JOIN mm_files     AS VideoFile       ON VideoFile.id = Rel.file_id
                                               AND VideoFile.IsVideoFile = 1
  LEFT JOIN mm_coleccoes AS VideoCollection ON VideoFile.coleccao_id = VideoCollection.id
WHERE 
  Destaque.area_id = 1
  AND Noticia.paraPublicacao = 1 
  AND Noticia.publicacao <= NOW()   
ORDER BY 
  Destaque.destaque

我的想法是：

您最多需要一个音频文件和一个视频文件。每个Noticia有几个文件可用，因此您需要确保每个类型最多一个文件进入连接。这也意味着您必须两次加入ntNoticias_mmFiles表 - 每种类型一次。

这是连接条件中的子查询应该执行的操作：为每种文件类型选择一行。从那里继续你LEFT JOIN其余的数据，就像你已经做的那样。

Answer 3

您可能希望将该连接查询优化到视图中。这是一个很大的查询，并且有很多连接，它的效率会非常低。此外，一个视图可以帮助您调试连接，并且基本上可以通过允许您分别编写连接（在视图中）和WHERE子句（在视图中选择）来简化，这可以帮助调试查询。

MySQL在同一个表上有2个LEFT JOIN

3 个答案: