优化MySQL查询

Question

我们有一个查询当前正在查杀我们的数据库,我知道必须有一种方法来优化它.我们有3个表:

1. items - 项目表,其中每个项目都有关联的object_id,length,difficulty_rating,rating,avg_rating和status
2. lists - 列表,基本上是我们用户创建的项目列表
3. list_items - 包含2列的表:list_id,item_id

我们一直在使用以下查询来显示一个简单的HTML表,该表显示每个列表以及与列表相关的一些属性,包括所包含列表项的属性的平均值:

select object_id, user_id, slug, title, description, items, 
       city, state, country, created, updated,
       (select AVG(rating) from items
          where object_id IN 
              (select object_id from list_items where list_id=lists.object_id) 
            AND status="A"
       ) as 'avg_rating',
       (select AVG(avg_rating) from items
          where object_id IN 
              (select object_id from list_items where list_id=lists.object_id) 
            AND status="A"
       ) as 'avg_avg_rating',
       (select AVG(length) from items 
          where object_id IN 
              (select object_id from list_items where list_id=lists.object_id) 
            AND status="A"
       ) as 'avg_length',
       (select AVG(difficulty_rating) from items 
          where object_id IN
              (select object_id from list_items where list_id=lists.object_id) 
            AND status="A"
       ) as 'avg_difficulty' 
    from lists
    where user_id=$user_id AND status="A" 
    order by $orderby LIMIT $start,$step

我们之所以没有在1个查询中解析这个以获取所有列表和后续查找以提取每个列表的平均值是因为我们希望用户能够对averages列进行排序(即'按avg_difficulty排序' ).

希望我的解释是有道理的.必须有一个更有效的方法来做到这一点,我希望那里的MySQL大师可以指出我正确的方向.谢谢!

Answer 1

看起来你可以用连接替换所有子查询:

SELECT     l.object_id,
           l.user_id,
           <other columns from lists>
           AVG(i.rating) as avgrating,
           AVG(i.avg_rating) as avgavgrating,
           <other averages>
FROM       lists l
LEFT JOIN  list_items li 
ON         li.list_id = l.object_id
LEFT JOIN  items i 
ON         i.object_id = li.object_id
           AND i.status = 'A'
WHERE      l.user_id = $user_id AND l.status = 'A' 
GROUP BY   l.object_id, l.user_id, <other columns from lists>

这将为数据库引擎节省大量工作.