如何收集每个 parent 的最大记录数

Question

我目前正在做一个 e-commerce 项目，我想创建一个登录页面来显示每个产品类别，每个类别有 4 个最新产品。我正在使用 MySQL，普通查询。

我的数据库相当简单，有一个产品 table 和一个 product_category table。当然有一个 link 从产品 table 到 product_category table: product_category_id.

编辑：我创建了一个数据库 fiddle： https://www.db-fiddle.com/f/fmy7mBfZ2vcu7hhumU59ou/0

当然，排序很简单，但我只需要每个 category_id 的 4 个产品结果。

在 product_category table 上方有一个 product_main_category table，但我认为这与此示例无关。

Answer 1

您可以在 mysql 5.7

中执行此操作

SELECT
    t.`id`
    , t.`product_category_id` 
    ,t.`name`, t.`slug`
    , t.`description`
    , t.`price`
    , t.`created_at`
    , t.`updated_at`
FROM
(
    SELECT 

        `id`, `name`, `slug`, `description`, `price`, `created_at`, `updated_at`,
        @row_num:=CASE WHEN @p_id = product_category_id THEN @row_num + 1 ELSE 1 END AS rn,
        @p_id:=product_category_id as product_category_id

    FROM
        products,(SELECT @p_id := 0) a1,(SELECT @row_num := 0) a2
    ORDER BY product_category_id, created_at
) t inner join product_categories pc on t.product_category_id = pc.id
WHERE t.rn <=4
ORDER BY t.product_category_id, t.id;

这给你以下结果

id | product_category_id | name                 | slug                                                                  | description                                                                                                                                                                          |  price | created_at          | updated_at         
-: | ------------------: | :------------------- | :-------------------------------------------------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -----: | :------------------ | :------------------
17 |                   1 | Sandra Thompson      | dolores-quidem-vel-sit-consequatur                                    | Porro dolorem voluptas sapiente at debitis et est. Sed molestias qui minima enim. Et nemo ad sed voluptate recusandae optio quaerat. Est occaecati eos non nostrum voluptatem porro. | 103.96 | 2020-03-12 23:38:50 | 2020-04-07 18:38:35
38 |                   1 | Mr. Domenic Kozey    | a-vero-quis-facilis                                                   | Reprehenderit fugit voluptatibus illo hic ut commodi. Deserunt maiores totam esse at voluptas et. Iusto et harum animi tenetur iste voluptatem.                                      | 195.03 | 2020-03-09 04:10:12 | 2020-04-07 18:38:35
46 |                   1 | Rosario Schmitt      | eveniet-a-rerum-sapiente-iure-in-et                                   | Nisi delectus labore autem qui dolores beatae accusamus. Libero totam consectetur aut rerum atque.                                                                                   | 126.25 | 2020-03-13 02:12:21 | 2020-04-07 18:38:35
48 |                   1 | Kaela Lueilwitz V    | ipsum-laborum-beatae-quia-repellat-hic-maxime                         | Magni non inventore quos. Et quasi itaque id. Corrupti sed maiores rem in.                                                                                                           |  43.01 | 2020-03-11 00:57:50 | 2020-04-07 18:38:35
 6 |                   2 | Jeremie Schneider    | sit-autem-molestiae-eveniet                                           | Corporis quidem aut nostrum suscipit in minima dicta autem. Nostrum porro dicta quo ut vero inventore enim. Repudiandae architecto voluptatem temporibus illo eum.                   |   3.77 | 2020-03-11 08:06:41 | 2020-04-07 18:38:35
27 |                   2 | Devan Hickle         | eaque-explicabo-aliquid-architecto-rem-odit-atque                     | Molestiae molestias eos voluptatem vero sunt id voluptatem neque. Molestiae nam cum aliquid. Odio non ab et non incidunt sed.                                                        |  60.81 | 2020-03-09 05:34:30 | 2020-04-07 18:38:35
32 |                   2 | Prof. Liana Torp     | quasi-eaque-soluta-sed-voluptates-odio                                | Voluptas est saepe voluptatem facere voluptates ratione. Esse rerum nihil deleniti nihil. Qui ut eum velit aut. Culpa rerum iure neque deserunt et.                                  |  67.77 | 2020-03-16 00:27:40 | 2020-04-07 18:38:35
33 |                   2 | Abdiel Schowalter    | quam-sapiente-omnis-aut-et-rerum-perferendis                          | Fuga dolor dolore et in rerum at. Voluptatibus quaerat ab eum qui necessitatibus maiores et. Quam consequatur ea quia maiores qui consequatur reiciendis.                            |   3.53 | 2020-03-08 01:00:48 | 2020-04-07 18:38:35
20 |                   3 | Mr. Eleazar Mante IV | voluptatem-consequatur-consequuntur-qui-veritatis-provident-dolor-quo | Illum sed eaque corrupti fuga. Sunt deserunt et iusto possimus id hic totam. Et voluptatem tempora nihil consequuntur voluptatem sequi quasi.                                        |  91.52 | 2020-03-08 07:44:49 | 2020-04-07 18:38:35
23 |                   3 | Fredy Morar          | error-aut-excepturi-fuga-aperiam-sunt-illum-ea                        | Libero non numquam optio. Nam aut reiciendis necessitatibus voluptates. Molestias officia in est tenetur.                                                                            | 181.54 | 2020-03-30 05:25:37 | 2020-04-07 18:38:35
36 |                   3 | Albina Hansen        | nam-et-labore-ipsum-quidem-aut                                        | Quo sequi consectetur quo amet quos. Eaque et velit aut quasi. Dolores incidunt autem blanditiis officiis dolor accusamus.                                                           |  97.39 | 2020-03-26 03:35:56 | 2020-04-07 18:38:35
44 |                   3 | Kenyon Mertz         | ut-aliquam-aut-modi-sit-sunt-sint-dignissimos                         | Dolore in corrupti sit et quis aperiam quia. Aspernatur aspernatur consequuntur voluptatem repellat rerum. Minima aliquam dolorem recusandae numquam voluptates.                     |  47.23 | 2020-03-21 10:29:19 | 2020-04-07 18:38:35

db<>fiddle here

结果已排序，因此您可以看到每个类别有 4 行。

因此应该检查或调整此顺序和内部选择

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