MySQL中没有子查询字段的模式计算? [英] Mode calculation without a subquery field in MySQL?

问题描述

在我的应用程序中，每个产品组都有很多产品，每个产品都有一个制造商.这些关系由 MySQL 存储在带有 id 字段的 InnoDB 表 product_groups 和带有 id 字段的 products 中，product_group 和 manufacturer 字段.

In my application, each product group has many products, and each product has one manufacturer. These relations are stored by MySQL in InnoDB tables product_groups with an id field, and products with id, product_group and manufacturer fields.

有没有办法找到每个产品组中最常见的制造商，而无需选择子查询?

Is there a way to find the most common manufacturer in each product group, without resorting to selecting subqueries?

这就是我目前的做法:

SELECT product_groups.id,
  (
    SELECT manufacturer FROM products
    WHERE product_group = product_groups.id
    GROUP BY manufacturer
    ORDER BY count(*) DESC
    LIMIT 1
  ) manufacturer_mode
FROM product_groups;

推荐答案

试试这个解决方案:

SELECT
    a.product_group,
    SUBSTRING_INDEX(GROUP_CONCAT(a.manufacturer ORDER BY a.occurrences DESC SEPARATOR ':::'), ':::', 1) AS manufacturer_mode
FROM
    (
        SELECT
            aa.product_group,
            aa.manufacturer,
            COUNT(*) AS occurrences
        FROM
            products aa
        GROUP BY
            aa.product_group,
            aa.manufacturer
    ) a
GROUP BY
    a.product_group

说明:

这仍然使用子查询的一种形式，但它只执行一次，而不是像在原始示例中那样逐行执行.

Explanation:

This still uses a form of subquery, but one which executes only once as opposed to one that executes on a row-by-row basis such as in your original example.

首先选择product_group ID、制造商以及制造商出现在每个特定组中的次数.

It works by first selecting the product_group id, the manufacturer, and the count of how many times the manufacturer appears for each particular group.

FROM 子选择在执行后会是这个样子(这里只是补数据):

The FROM sub-select will look something like this after execution (just making up data here):

product_group   |   manufacturer   |    occurrences
---------------------------------------------------
1               |   XYZ            |    4
1               |   Test           |    2
1               |   Singleton      |    1
2               |   Eloran         |    2
2               |   XYZ            |    1

现在我们有了子选择结果，我们需要为每个产品组挑选出 occurrences 字段中具有最大值的行.

Now that we have the sub-select result, we need to pick out the row that has the maximum in the occurences field for each product group.

在外部查询中，我们再次按 product_group 字段对子选择进行分组，但这一次，仅 product_group 字段.现在，当我们在这里执行 GROUP BY 时，我们可以在 MySQL 中使用一个非常引人注目的函数，称为 GROUP_CONCAT，我们可以使用它以我们想要的任何顺序将制造商连接在一起.

In the outer query, we group the subselect once again by the product_group field, but this time, only the product_group field. Now when we do our GROUP BY here, we can use a really compelling function in MySQL called GROUP_CONCAT which we can use to concatenate the manufacturers together and in any order we want.

...GROUP_CONCAT(a.manufacturer ORDER BY a.occurrences DESC SEPARATOR ':::'...

我们在这里所做的是将按照 product_group id 分组在一起的制造商连接在一起，ORDER BY a.occurrences DESC 确保制造商拥有最多外观出现在串联列表中首先.最后，我们用 ::: 分隔每个制造商.product_group 1 的结果如下:

What we are doing here is concatenating the manufacturers together that are grouped together per product_group id, the ORDER BY a.occurrences DESC makes sure that the manufacturer with the most appearances appears first in the concatenated list. Finally we are separating each manufacturer with :::. The result of this for product_group 1 will look like:

XYZ:::Test:::Singleton

XYZ 首先出现，因为它在 occurance 字段中具有最高值.我们只想要选择XYZ，所以我们将连接包含在SUBSTRING_INDEX中，这将允许我们只选择基于列表的第一个元素在 ::: 分隔符上.

XYZ appears first since it has the highest value in the occurance field. We only want to select XYZ, so we encase the concatenation within SUBSTRING_INDEX, which will allow us to only pick the first element of the list based on the ::: delimiter.

最终结果将是:

product_group    |    manufacturer_mode
---------------------------------------
1                |    XYZ
2                |    Eloran

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