mysql 树形查询三级

CREATE TABLE categories (
    id INT PRIMARY KEY,
    name VARCHAR(255),
    parent_id INT,
    FOREIGN KEY (parent_id) REFERENCES categories(id)
);

INSERT INTO categories (id, name, parent_id) VALUES
(1, 'Electronics', NULL),
(2, 'Computers', 1),
(3, 'Laptops', 2),
(4, 'Gaming Laptops', 3),
(5, 'Accessories', 1),
(6, 'Keyboards', 5);

WITH RECURSIVE category_tree AS (
    SELECT id, name, parent_id, 1 AS level
    FROM categories
    WHERE parent_id IS NULL
    UNION ALL
    SELECT c.id, c.name, c.parent_id, ct.level + 1
    FROM categories c
    JOIN category_tree ct ON c.parent_id = ct.id
    WHERE ct.level < 3
)
SELECT * FROM category_tree;