mysql Max有多个实例

Question

我正在寻找一种方法来适应寻找最大值和最小值的多个结果。我找到了上一个问题的链接: max Counts

给出的答案之一是:

SELECT  color_id, COUNT(color_id) totalCount
FROM    products 
WHERE   item_id = 1234 
GROUP   BY color_id 
HAVING  COUNT(color_id) =
(
  SELECT  COUNT(color_id) totalCount
  FROM    products 
  WHERE   item_id = 1234 
  GROUP   BY color_id 
  ORDER BY totalCount DESC
  LIMIT 1  
)

这种做法是否被接受，特别是对于大型数据库？如果有意义的话，上面的查询基本上不是在自身内部运行吗？

我有一个更复杂的查询，还需要找到 ma 和 min。我想优化它:

编辑:

SELECT `system_users`.`first`, `system_users`.`last`,  COUNT(`quotes`.`created_by`) as most_quotes
FROM `quotes` 
INNER JOIN `system_users`
ON `quotes`.`created_by` = `system_users`.`id`
where `system_users`.`store_id` = '$createdID' 
and `quotes`.`date_created` between '$startDate' and '$endDate' group by(`created_by`)
HAVING count(`quotes`.`created_by`) = 
(
SELECT COUNT(`quotes`.`created_by`)
FROM `quotes` 
INNER JOIN `system_users`
ON `quotes`.`created_by` = `system_users`.`id`
where `system_users`.`store_id` = '$createdID' 
and `quotes`.`date_created` between '$startDate' and '$endDate' group by(`created_by`) ORDER BY count(`created_by`) DESC limit 1
)
OR
(
SELECT COUNT(`quotes`.`created_by`)
FROM `quotes` 
INNER JOIN `system_users`
ON `quotes`.`created_by` = `system_users`.`id`
where `system_users`.`store_id` = '$createdID'
and `quotes`.`date_created` between '$startDate' and '$endDate' group by(`created_by`) ORDER BY count(`created_by`) ASC limit 1
)
ORDER BY most_quotes ASC

我正在尝试寻找不同的方法来找到最大值和最小值，但到目前为止还没有运气。对此的任何更多帮助将不胜感激谢谢MC

Answer 1

这是一个坏主意 - 在大型数据库上使用 HAVING。而且，此外，你的问题可以这样解决(我有MySQL 5.5版本):

SELECT  
  color_id, 
  COUNT(color_id) AS totalCount
FROM    
  products 
WHERE   
  item_id = 1234 
GROUP BY 
  color_id 
ORDER BY 
  totalCount DESC 
LIMIT 1

HAVING 的问题在于它是在整个查询完成后执行的，即存储引擎已经工作，因此无法对 HAVING 进行索引或其他优化条件 - 因此，它可以被视为完整的结果集扫描。

感谢@GordonLinoff，我发现这并不完全是您想要的东西。如果您试图找到所有相应的行，您最好按照戈登的建议行事。

虽然我找到了另一种方法来解决这个问题，但它可能只比带有 HAVING 的原始变体好一些(而且 - 更好，因为存储引擎两次都会涉及)

SELECT
  first.color_id, 
  first.rows_count 
FROM 
  (SELECT color_id, COUNT(1) AS rows_count FROM products WHERE item_id=1234 GROUP BY color_id) AS first 
  LEFT JOIN
  (SELECT color_id, COUNT(1) AS rows_count FROM products WHERE item_id=1234 GROUP BY color_id ORDER BY rows_count DESC LIMIT 1) AS second 
    ON first.rows_count=second.rows_count 
WHERE second.rows_count IS NOT NULL;

我还有带有变量的变体(类似于戈登的变体)。因此您可以在这些选项之间进行选择。