具有多个实例的mysql Max
我正在寻找一种方法,以适应查找最大值和最小值时出现的多个结果。我找到了与上一个问题的链接: 其中一个答案是:具有多个实例的mysql Max,mysql,count,max,having,Mysql,Count,Max,Having,我正在寻找一种方法,以适应查找最大值和最小值时出现的多个结果。我找到了与上一个问题的链接: 其中一个答案是: 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
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
)
这是一种公认的做法,特别是对于大型数据库?如果这有意义的话,上面的查询不是基本上在自身内部运行吗
我有一个更复杂的查询,也需要找到ma和min。我想优化它:
编辑:
我正在尝试用不同的方法来寻找最大值和最小值,但到目前为止运气都不好。如果您能在这方面提供更多帮助,我们将不胜感激
谢谢
mc在大型数据库上使用have是个坏主意。此外,您的问题可以通过以下方式解决: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
have的问题是,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have条件进行索引或其他优化,因此,它可以被视为完整的结果集扫描
多亏了@GordonLinoff,我发现这并不是你想要的。如果要查找所有对应的行,最好按照Gordon的建议进行操作
虽然我找到了另一种方法来解决这个问题,但它可能只比最初的版本好几点,因为存储引擎会同时涉及到这两个方面
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;
我还有一个变量和Gordon的变量相似。因此,您可以在这些选项中进行选择。您的问题的答案是,在MySQL中,使用一个查询有多种方法可以实现这一点 一种方法是你问题中的方法。另一种方法是使用变量:
select color_id, totalcount
from (SELECT color_id, COUNT(color_id) as totalCount,
@maxcount = if(@maxcount < count(color_id), count(color_id), @maxcount)
FROM products cross join
(select @maxcount := 0) const
WHERE item_id = 1234
GROUP BY color_id
) t
where totalCount = @maxcount;
select color_id, totalcount
from (SELECT color_id, COUNT(color_id) as totalCount,
@maxcount = if(@maxcount < count(color_id), count(color_id), @maxcount),
@mincount = if(@mincount > count(color_id) or @mincount = -1, count(color_id), @mincount)
FROM products cross join
(select @maxcount := -1, @mincount := -1) const
WHERE item_id = 1234
GROUP BY color_id
) t
where totalCount in (@mincount, @maxcount);
但是,MySQL不支持这些
编辑:
如果要使用变量计算最大值和最小值:
select color_id, totalcount
from (SELECT color_id, COUNT(color_id) as totalCount,
@maxcount = if(@maxcount < count(color_id), count(color_id), @maxcount)
FROM products cross join
(select @maxcount := 0) const
WHERE item_id = 1234
GROUP BY color_id
) t
where totalCount = @maxcount;
select color_id, totalcount
from (SELECT color_id, COUNT(color_id) as totalCount,
@maxcount = if(@maxcount < count(color_id), count(color_id), @maxcount),
@mincount = if(@mincount > count(color_id) or @mincount = -1, count(color_id), @mincount)
FROM products cross join
(select @maxcount := -1, @mincount := -1) const
WHERE item_id = 1234
GROUP BY color_id
) t
where totalCount in (@mincount, @maxcount);
这可能是一个更有效的查询,但它不能回答问题。该问题特别指出,多个颜色标识可能满足最大条件。这只返回一个值。Hm@GordonLinoff,这是正确的-因为上面的查询只返回一行。我会改正的。谢谢你的回复。虽然我的问题比那复杂一点。就目前而言,这就是我所拥有的,用于查找最大值和最小值:请参见上面的编辑,我无法将我的查询添加为注释。我想容纳一个以上的最大值和最小值,但不知道how@GordonLinoff你能为我附加的报价提供一个解决方案,找出最大值和最小值吗?谢谢你能建议我修改一下上面的问题吗?ThanksI运行此查询,并返回空列