限制每个类别的结果数量，需要建议答案

【问题标题】：Limiting number of result from each category, advise needed限制每个类别的结果数量，需要建议
【发布时间】：2013-06-11 03:45:49
【问题描述】：

我有这个代码

<?php
function Random($numchar)  
{  
$categories = "83 76 168 102";  
$array=explode(" ",$categories);  
shuffle($array);  
$newstring = implode($array,",");
return substr($newstring, 0, $numchar);  

}
$numbers = Random(216);
$res = str_replace(",","','",$numbers);
$cats = '\''.$res.'\'';
$news = mysql_query("SELECT 
item.id AS itemid, 
item.cid AS itemcid, 
SUBSTRING_INDEX(item.url, '/', 3) AS itemurl, 
item.title AS itemtitle, 
item.top AS itemtop, 
images.id AS imgid, 
images.img AS imagesimg, 
images.item_id AS imagesitemid,
folders.id AS foldersid,
channels.id AS channelsid,
channels.parent AS channelsparent
FROM item, images, folders, channels 
WHERE 
item.id = images.item_id
AND item.cid = channels.id
AND channels.parent = folders.id
AND channels.parent = '$cat'
AND FIELD(item.cid, $cats)
ORDER BY 
itemtop DESC, RAND(), images.item_id DESC 
LIMIT 20");

?>

每次刷新页面时，代码都会从变量 $categories 中定义的类别之一返回 20 项。我只想显示每个类别的 5 个项目。是否可以像我预期的那样使用单个查询？我想腾出空间，以便可以平等地显示所有类别的项目。

还有其他方法可以优化查询吗？

【问题讨论】：

为什么不是LIMIT 5？
5 将显示来自单个类别的 5 个项目，我必须为其他类别创建 3 个其他查询。问题是当类别变成 50 个类别时，我无法在同一页面上编写 50 个查询。
您也许可以通过在同一个表上使用多个连接查询来做到这一点 - 每个类别一个。

标签： php mysql shuffle

【解决方案1】：

为了从每个类别中获取 5 个项目，您必须对每个类别运行单独的子查询，限制为 5，然后将它们组合在一起。这是一个例子：

SELECT
  *
FROM (
  (SELECT
    item.id AS itemid, 
    item.cid AS itemcid, 
    SUBSTRING_INDEX(item.url, '/', 3) AS itemurl, 
    item.title AS itemtitle, 
    item.top AS itemtop, 
    images.id AS imgid, 
    images.img AS imagesimg, 
    images.item_id AS imagesitemid,
    folders.id AS foldersid,
    channels.id AS channelsid,
    channels.parent AS channelsparent
  FROM
    item,
    images,
    folders,
    channels
  WHERE 
    item.id = images.item_id
    AND item.cid = channels.id
    AND channels.parent = folders.id
    AND channels.parent = '$cat[0]'
    AND FIELD(item.cid, $cats[0])
  LIMIT 5)

  UNION ALL

  (SELECT
    item.id AS itemid, 
    item.cid AS itemcid, 
    SUBSTRING_INDEX(item.url, '/', 3) AS itemurl, 
    item.title AS itemtitle, 
    item.top AS itemtop, 
    images.id AS imgid, 
    images.img AS imagesimg, 
    images.item_id AS imagesitemid,
    folders.id AS foldersid,
    channels.id AS channelsid,
    channels.parent AS channelsparent
  FROM
    item,
    images,
    folders,
    channels
  WHERE 
    item.id = images.item_id
    AND item.cid = channels.id
    AND channels.parent = folders.id
    AND channels.parent = '$cat[1]'
    AND FIELD(item.cid, $cats[1])
  LIMIT 5)

  UNION ALL

  (SELECT
    item.id AS itemid, 
    item.cid AS itemcid, 
    SUBSTRING_INDEX(item.url, '/', 3) AS itemurl, 
    item.title AS itemtitle, 
    item.top AS itemtop, 
    images.id AS imgid, 
    images.img AS imagesimg, 
    images.item_id AS imagesitemid,
    folders.id AS foldersid,
    channels.id AS channelsid,
    channels.parent AS channelsparent
  FROM
    item,
    images,
    folders,
    channels
  WHERE 
    item.id = images.item_id
    AND item.cid = channels.id
    AND channels.parent = folders.id
    AND channels.parent = '$cat[2]'
    AND FIELD(item.cid, $cats[2])
  LIMIT 5)
)sub_query

ORDER BY 
  itemtop DESC, RAND(), sub_query.item_id DESC

您可能希望在子查询中添加 order by 以定义为每个类别返回的 5 个。您可能还需要修改 $cat 的定义方式。希望这会有所帮助。

【讨论】：

@D_Guy13 如果需要，您可以使用 foreach($cat as $value){ 为每个类别编写此查询，然后使用 $query .="beginning of query" 预先挂起和后挂查询的开头和结尾执行$news = mysql_query($query);

【解决方案2】：

您可以尝试使用用户变量...不确定 php 如何/是否会处理它，但是使用原始 sql 类似这样的东西应该可以工作（未经测试，但您应该明白这一点，请参阅 http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/ 了解更多详细信息):

set @num := 0, @cid := '';

SELECT 
item.id AS itemid, 
item.cid AS itemcid, 
SUBSTRING_INDEX(item.url, '/', 3) AS itemurl, 
item.title AS itemtitle, 
item.top AS itemtop, 
images.id AS imgid, 
images.img AS imagesimg, 
images.item_id AS imagesitemid,
folders.id AS foldersid,
channels.id AS channelsid,
channels.parent AS channelsparent,
@num := if(@cid = item.cid, @num + 1, 1) as row_number,
@cid := cid as dummy
FROM item, images, folders, channels 
WHERE 
item.id = images.item_id
AND item.cid = channels.id
AND channels.parent = folders.id
AND channels.parent = '$cat'
AND FIELD(item.cid, $cats)
ORDER BY 
itemtop DESC GROUP, RAND(), images.item_id DESC
GROUP BY item.id, item.cid, item.title
having row_number <= 5;

【讨论】：