如何從表中獲取所有沒有父節點的節點？

Question

如何從表中獲取沒有父節點的所有節點？

我正在使用（閉包表）模板。

我需要獲取所有沒有父節點的節點。

考慮這樣一個事實，即每個節點都引用自己。 也就是說，每個節點本身都是與其自身相關的父節點和子節點。

它看起來像這樣。

下面我展示了一個測試數據庫。

-- phpMyAdmin SQL Dump
-- version 5.1.0
-- https://www.phpmyadmin.net/
--
-- Host: localhost
-- Generation Time: Oct 15, 2021 at 06:30 PM
-- Server version: 8.0.25
-- PHP Version: 8.0.3

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
START TRANSACTION;
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `closure`
--

-- --------------------------------------------------------

--
-- Table structure for table `category_name`
--

CREATE TABLE `category_name` (
  `id` bigint NOT NULL,
  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  `level` int DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

--
-- Dumping data for table `category_name`
--

INSERT INTO `category_name` (`id`, `name`, `level`) VALUES
(1, 'Electronics', 0),
(2, 'TV sets', 0),
(3, 'Sensory', 0),
(4, 'Backlit', 0),
(5, 'On wheels', 0),
(6, 'Anti-glare', 0),
(7, 'Super thin', 0),
(8, 'Wall', 0),
(9, 'Telephone', 0),
(10, 'Shell', 0),
(11, 'Button', 0),
(12, 'Sensory', 0),
(13, 'Retractable', 0),
(14, 'Auto', 0),
(15, 'Manual', 0),
(16, 'For home', 0),
(17, 'For bathroom', 0),
(18, 'Rug', 0),
(19, 'With animals', 0),
(20, 'Obstruction', 0),
(21, 'Soap dish ', 0),
(22, 'With holes', 0),
(23, 'Transparent', 0),
(24, 'For kitchen', 0),
(25, 'Pans', 0),
(26, 'With handles', 0),
(27, 'Non-stick', 0),
(28, 'pans', 0),
(29, 'Steam', 0),
(30, 'With lids', 0),
(31, 'Test', 0);

-- --------------------------------------------------------

--
-- Table structure for table `tree_path`
--

CREATE TABLE `tree_path` (
  `children` bigint NOT NULL,
  `parent` bigint NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

--
-- Dumping data for table `tree_path`
--

INSERT INTO `tree_path` (`children`, `parent`) VALUES
(1, 1),
(2, 1),
(3, 1),
(4, 1),
(5, 1),
(6, 1),
(7, 1),
(8, 1),
(9, 1),
(10, 1),
(11, 1),
(12, 1),
(13, 1),
(14, 1),
(15, 1),
(2, 2),
(3, 2),
(4, 2),
(5, 2),
(6, 2),
(7, 2),
(8, 2),
(3, 3),
(4, 3),
(5, 3),
(4, 4),
(5, 5),
(6, 6),
(7, 6),
(8, 6),
(7, 7),
(8, 8),
(9, 9),
(10, 9),
(11, 9),
(12, 9),
(13, 9),
(14, 9),
(15, 9),
(10, 10),
(11, 10),
(12, 10),
(11, 11),
(12, 12),
(13, 13),
(14, 13),
(15, 13),
(14, 14),
(15, 15),
(16, 16),
(17, 16),
(18, 16),
(19, 16),
(20, 16),
(21, 16),
(22, 16),
(23, 16),
(24, 16),
(25, 16),
(26, 16),
(27, 16),
(28, 16),
(29, 16),
(30, 16),
(17, 17),
(18, 17),
(19, 17),
(20, 17),
(21, 17),
(22, 17),
(23, 17),
(18, 18),
(19, 18),
(20, 18),
(19, 19),
(20, 20),
(21, 21),
(22, 21),
(23, 21),
(22, 22),
(23, 23),
(24, 24),
(25, 24),
(26, 24),
(27, 24),
(28, 24),
(29, 24),
(30, 24),
(25, 25),
(26, 25),
(27, 25),
(26, 26),
(27, 27),
(28, 28),
(29, 28),
(30, 28),
(29, 29),
(30, 30);

--
-- Indexes for dumped tables
--

--
-- Indexes for table `category_name`
--
ALTER TABLE `category_name`
  ADD PRIMARY KEY (`id`);

--
-- Indexes for table `tree_path`
--
ALTER TABLE `tree_path`
  ADD PRIMARY KEY (`children`,`parent`),
  ADD KEY `FK_PARENT` (`parent`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `category_name`
--
ALTER TABLE `category_name`
  MODIFY `id` bigint NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=33;

--
-- Constraints for dumped tables
--

--
-- Constraints for table `tree_path`
--
ALTER TABLE `tree_path`
  ADD CONSTRAINT `FK_CHILDREN` FOREIGN KEY (`children`) REFERENCES `category_name` (`id`) ON DELETE CASCADE,
  ADD CONSTRAINT `FK_PARENT` FOREIGN KEY (`parent`) REFERENCES `category_name` (`id`) ON DELETE CASCADE;
COMMIT;

/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;

或許答案會是這樣。

  +----+--------------+-------+----------+--------+
  | id | name         | level | children | parent |
  +----+--------------+-------+----------+--------+
  |  1 | Electronics  |     0 |      1   |     1  |
  |  16| For home     |     0 |      16  |     16 |
  +----+--------------+-------+----------+--------+

Answer 1

我會使用NOT EXISTS ()檢查tree_path表中是否沒有父行（除了它本身）。

SELECT
  *
FROM
  category_name   c
WHERE
  NOT EXISTS (
    SELECT *
      FROM tree_path   t
     WHERE t.parent != c.id
       AND t.children = c.id
  )
  AND EXISTS (
    SELECT *
      FROM tree_path   t
     WHERE t.parent = c.id
       AND t.children = c.id
  )

演示： https : //dbfiddle.uk/?rdbms=mysql_8.0&fiddle=55698aa8b51f99d42db63feda15148a7

（編輯以排除不在 tree_path 中的類別）