为什么不在面包屑中显示所有后代类别？答案

【问题标题】：Why isn't it showing all descendant categories in breadcrumbs?为什么不在面包屑中显示所有后代类别？
【发布时间】：2022-01-21 06:15:08
【问题描述】：

按产品类别搜索时，我需要显示其所有父类别。

当有多个级别时，它只显示两个结果。

CREATE TABLE `category` (
  `id` int NOT NULL,
  `parent_category_id` int,
  `name` varchar(20) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL
) ENGINE=InnoDB;

INSERT INTO `category` (`id`, `parent_category_id`, `name`) VALUES
(1, NULL, 'Male'),
(2, 1, 'T-shirts'),
(3, 1, 'Shoes'),
(4, 2, 'Tank top'),
(5, 2, 'Basic shirts');

SELECT CONCAT(t1.name, ' > ', t2.name) as breadcrumb
FROM category AS t1
LEFT JOIN category AS t2 ON t2.parent_category_id = t1.id
WHERE t2.id = 4; #( 4 or 5 )

结果： T 恤 > 背心

预期结果：男>T恤>背心

不显示“男性”类别

【问题讨论】：

检查这个post注意它应该是parent_category_id而不是subcategory_id

标签： mysql database recursion breadcrumbs

【解决方案1】：

嗯，你只加入一个级别。如果要加入任意关卡，可以使用递归 CTE。

WITH RECURSIVE
breadcrumbs
AS
(
SELECT 1 AS ordinality,
       c.name,
       c.subcategory_id
       FROM category AS c
       WHERE c.id = 4
UNION ALL
SELECT bc.ordinality + 1 AS ordinality,
       c.name,
       c.subcategory_id
       FROM breadcrumbs AS bc
            INNER JOIN category AS c
                       ON c.id = bc.subcategory_id
)
SELECT group_concat(bc.name
                    ORDER BY bc.ordinality DESC
                    SEPARATOR ' > ') AS breadcrumb
       FROM breadcrumbs AS bc;

db<>fiddle

【讨论】：

成功了，非常感谢！