Question

此查询有效..一次查询50个mysql数据库并返回最新的10个结果，但是..有没有办法不查询每个状态的整个数据库（我的例子中只列出了4个状态），只是获得前10名（通过timeStamp desc）并使这个查询更有效率？

$query = "    
SELECT imgURLThumb, timeStamp, title2, state, postId, title
FROM (
  SELECT imgURLThumb,timeStamp, title2, state, postId, title FROM db_washington.md_postings UNION ALL 
  SELECT imgURLThumb,timeStamp, title2, state, postId, title FROM db_westvirginia.md_postings UNION ALL 
  SELECT imgURLThumb,timeStamp, title2, state, postId, title FROM db_wisconsin.md_postings UNION ALL 
  SELECT imgURLThumb,timeStamp, title2, state, postId, title FROM db_wyoming.md_postings 

) allposts where imgURLThumb <> 'images/nopicture.png' order by timeStamp DESC LIMIT 0 , 10";

Answer 1

您应该重新设计数据库，以便所有帖子都在一个数据库中，以便更容易编写此查询。为每个州建立一个数据库看起来很糟糕。您应该为所有帖子创建一个表md_postings，其中一个字段为State。

如果那是不可能的，那么我认为你有一个权衡：

简洁易读的SQL或
表现良好。

如果你想要更好的表现，试试这个：

SELECT imgURLThumb, timeStamp, title2, state, postId, title
FROM (
    (SELECT imgURLThumb,timeStamp, title2, state, postId, title
    FROM db_washington.md_postings
    WHERE imgURLThumb <> 'images/nopicture.png'
    ORDER BY timeStamp DESC
    LIMIT 10)

    UNION ALL

    (SELECT imgURLThumb,timeStamp, title2, state, postId, title
    FROM db_westvirginia.md_postings
    WHERE imgURLThumb <> 'images/nopicture.png'
    ORDER BY timeStamp DESC
    LIMIT 10)

    UNION ALL

    (SELECT imgURLThumb,timeStamp, title2, state, postId, title
    FROM db_wisconsin.md_postings
    WHERE imgURLThumb <> 'images/nopicture.png'
    ORDER BY timeStamp DESC
    LIMIT 10)

    UNION ALL

    (SELECT imgURLThumb,timeStamp, title2, state, postId, title
    FROM db_wyoming.md_postings
    WHERE imgURLThumb <> 'images/nopicture.png'
    ORDER BY timeStamp DESC
    LIMIT 10)
) AS allposts
WHERE imgURLThumb <> 'images/nopicture.png'
ORDER BY timeStamp DESC LIMIT 10

使用UNION ALL时如何简化此MySQL查询？

1 个答案: