MySQL的左连接右表最大值

Question

我想选择只有一个评论的每一张照片，我想这一评论是具有最大ID

我曾尝试以下操作：

SELECT 
    p.id, 
    p.title, 
    MAX(c.id), 
    c.comment 
FROM tb_photos AS p 
    LEFT JOIN tb_comments AS c ON p.id=c.photos_id. 

它似乎在工作，但我想知道是否有更好的方法来做到这一点？

Answer 1

你需要在每张照片应用MAX（注释ID）（假设注释ID是自动递增，因此总是最新添加到表）

select 
     p.*, 
     tbc.Comment 
    from 
     tb_photos p 
     LEFT JOIN (select c.photos_id, 
          max(c.id) lastCommentPerPhoto 
         from 
          tb_comments c 
         group by 
          c.photos_id 
         order by 
          c.Photos_id) LastPhotoComment 
      on p.id = LastPhotoComment.photos_id 
      LEFT JOIN tb_comments tbc 
       on LastPhotoComment.LastCommentPerPhoto = tbc.id 

Answer 2

你也可以做到这一点与交叉连接：

select 
     p.*, 
     LastPhotoComment.Comment 
    from 
     tb_photos p 
     cross join (select top 1 c.Comment 
         from 
          tb_comments c 
         where 
          c.photos_id = p.id 
         order by 
          c.id DESC) LastPhotoComment