MySQL:如何将名称连接为树(父子类别)

CREATE TABLE categories (
    category_id INT PRIMARY KEY,
    category_name VARCHAR(50),
    parent_category_id INT
);

INSERT INTO categories (category_id, category_name, parent_category_id)
VALUES
    (1, '电子产品', NULL),
    (2, '手机', 1),
    (3, '电脑', 1),
    (4, '家用电器', NULL),
    (5, '厨房电器', 4),
    (6, '卫浴电器', 4);

SELECT
    CONCAT(REPEAT('    ', LEVEL - 1), c.category_name) AS category_tree
FROM
    categories AS c
JOIN
    (
    SELECT
        category_id,
        category_name,
        parent_category_id,
        @level := @level + 1 AS LEVEL
    FROM
        categories
    JOIN
        (SELECT @level := 0) AS init
    WHERE
        parent_category_id IS NULL
    UNION ALL
    SELECT
        c.category_id,
        c.category_name,
        c.parent_category_id,
        @level := @level + 1 AS LEVEL
    FROM
        categories AS c
    JOIN
        (
        SELECT
            @level := 0
        ) AS init
    JOIN
        categories AS p ON c.parent_category_id = p.category_id
    ) AS t ON c.category_id = t.category_id
ORDER BY
    category_id;