mysql – 为什么这个group_concat无法正常工作？

我不明白为什么这个GROUP_CONCAT不起作用,就外部查询而言,返回了3行所以我想通过它来组合group_concat但它不喜欢它…

http://sqlfiddle.com/#!2/24764/3

CREATE TABLE nested_category (
        category_id INT AUTO_INCREMENT PRIMARY KEY,
        name VARCHAR(20) NOT NULL,
        lft INT NOT NULL,
        rgt INT NOT NULL
);

INSERT INTO nested_category VALUES(1,'ELECTRONICS',1,20),(2,'TELEVISIONS',2,9),(3,'TUBE',3,4),
 (4,'LCD',5,6),(5,'PLASMA',7,8),(6,'PORTABLE ELECTRONICS',10,19),(7,'MP3 PLAYERS',11,14),(8,'FLASH',12,13),
 (9,'CD PLAYERS',15,16),(10,'2 WAY RADIOS',17,18);

SELECT GROUP_CONCAT(rs.category_id, ',')
FROM 
    (
        SELECT node.category_id, node.name, (COUNT(parent.name) - (sub_tree.depth + 1)) AS depth
        FROM nested_category AS node,
                nested_category AS parent,
                nested_category AS sub_parent,
                (
                        SELECT node.category_id, node.name, (COUNT(parent.name) - 1) AS depth
                        FROM nested_category AS node,
                                nested_category AS parent
                        WHERE node.lft BETWEEN parent.lft AND parent.rgt
                            AND node.name = 'PORTABLE ELECTRONICS'
                        GROUP BY node.name
                        ORDER BY node.lft
                )AS sub_tree
        WHERE node.lft BETWEEN parent.lft AND parent.rgt
                AND node.lft BETWEEN sub_parent.lft AND sub_parent.rgt
                AND sub_parent.name = sub_tree.name
        GROUP BY node.name
        HAVING depth = 1
        ORDER BY node.lft
    ) as rs
GROUP BY rs.category_id

解决方法:

两件事情：

第一步：将GROUP_CONCAT(rs.category_id,’,’)更改为GROUP_CONCAT(rs.category_id)

逗号是默认分隔符,from the docs你可以看到,如果你想改变类似的分隔符
GROUP_CONCAT(rs.category_id SEPARATOR’|’)

第二：删除最后一个：
GROUP BY rs.category_id

如果按每个category_id进行分组,则表示每个category_id都在其自己的集合中,因此group_concat每行只有一个类别.

http://sqlfiddle.com/#!2/24764/7

标签：sql,mysql,database,hierarchical-data
来源： https://codeday.me/bug/20190723/1510004.html