mysql - 查询以从新闻提要中获取评论

Question

当一个人发布新闻提要并且其他用户成为 friend 时，必须同时选择此人和他/她 friend 的新闻提要根据判断是否为好友，必须选择对动态消息的评论

这是我的下表结构

下面的查询成功地从这个人以及这个人的 friend 那里获取了具有有效用户名和用户照片的新闻提要

$data = $this->db->query("
SELECT DISTINCT(ft.ID) as ID, ft.userid, ft.content, ft.timestamp, 
                ft.likes, ft.comments, u.username, u.avatar 
    FROM feed_item ft, users u 
    WHERE ft.userid = u.ID AND ft.userid 
    IN 
    (SELECT u.ID 
     FROM users u 
     WHERE 
        u.ID IN (SELECT uf.friendid FROM user_friends uf WHERE uf.status = '2' AND uf.userid = '".$this->user->info->ID."') 
     OR u.ID IN (SELECT uf.userid FROM user_friends uf WHERE uf.status = '2' AND uf.friendid = '".$this->user->info->ID."')
     OR u.ID = '".$this->user->info->ID."'
    ) 
ORDER BY ft.ID DESC")->result_array();

但是有一次我修改了查询以仅从该人的 friend 那里检索所有评论。

这导致获取具有空值的用户名和用户照片

$data = $this->db->query("
SELECT DISTINCT ft.ID as ID, ft.userid, ft.content, ft.timestamp, 
       ft.likes, ft.comments, ftc.comment, u.username, u.avatar 
FROM  feed_item_comment ftc
LEFT JOIN feed_item ft 
  ON ftc.postid = ft.ID 
 AND ftc.userid != '".$this->user->info->ID."' AND ftc.userid = ft.userid
LEFT JOIN user_friends uf 
  ON uf.friendid = ftc.userid 
LEFT JOIN users u 
  ON u.ID = uf.friendid 
 AND ft.userid  IN 
(SELECT u.ID 
 FROM users u 
 WHERE 
    u.ID IN (SELECT uf.friendid FROM user_friends uf WHERE uf.status = '2' AND uf.userid = '".$this->user->info->ID."') 
 OR u.ID IN (SELECT uf.userid FROM user_friends uf WHERE uf.status = '2' AND uf.friendid = '".$this->user->info->ID."')
 OR u.ID = '".$this->user->info->ID."'
) 
ORDER BY ft.ID DESC")->result_array();

在从对新闻提要的嵌套评论，以及来自此人及其 friend 的具有有效用户名和用户照片的帖子？

更新

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-- Generation Time: Oct 09, 2018 at 04:18 PM
-- Server version: 10.1.36-MariaDB
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/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `gamersapi`
--

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

--
-- Table structure for table `feed_item_comment`
--

CREATE TABLE `feed_item_comment` (
  `ID` int(11) NOT NULL,
  `postid` int(11) NOT NULL,
  `userid` int(11) NOT NULL,
  `comment` varchar(3000) NOT NULL,
  `timestamp` int(11) NOT NULL,
  `likes` int(11) NOT NULL,
  `commentid` int(11) NOT NULL,
  `replies` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

--
-- Dumping data for table `feed_item_comment`
--

INSERT INTO `feed_item_comment` (`ID`, `postid`, `userid`, `comment`, `timestamp`, `likes`, `commentid`, `replies`) VALUES
(1, 184, 1, 'comment', 1539080007, 0, 0, 0),
(2, 186, 14, 'VBVBVB', 1539084437, 0, 0, 0),
(3, 186, 14, 'VVV', 1539084448, 0, 0, 0),
(4, 187, 4, 'zzz', 1539084875, 0, 0, 0);

--
-- Indexes for dumped tables
--

--
-- Indexes for table `feed_item_comment`
--
ALTER TABLE `feed_item_comment`
  ADD PRIMARY KEY (`ID`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `feed_item_comment`
--
ALTER TABLE `feed_item_comment`
  MODIFY `ID` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=5;
COMMIT;

/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
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-- phpMyAdmin SQL Dump
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-- https://www.phpmyadmin.net/
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-- Host: 127.0.0.1
-- Generation Time: Oct 09, 2018 at 04:18 PM
-- Server version: 10.1.36-MariaDB
-- PHP Version: 7.2.10

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET AUTOCOMMIT = 0;
START TRANSACTION;
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
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--
-- Database: `gamersapi`
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-- --------------------------------------------------------

--
-- Table structure for table `feed_item`
--

CREATE TABLE `feed_item` (
  `ID` int(11) NOT NULL,
  `userid` int(11) NOT NULL,
  `content` text NOT NULL,
  `timestamp` time NOT NULL,
  `imageid` int(11) NOT NULL,
  `likes` int(11) NOT NULL,
  `comments` int(11) NOT NULL,
  `user_flag` int(11) NOT NULL,
  `likes_data` varchar(10) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

--
-- Dumping data for table `feed_item`
--

INSERT INTO `feed_item` (`ID`, `userid`, `content`, `timestamp`, `imageid`, `likes`, `comments`, `user_flag`, `likes_data`) VALUES
(1, 1, 'How are you', '00:00:00', 0, 0, 0, 0, 'like'),
(2, 1, 'How are you doing', '00:00:00', 0, 0, 0, 0, 'like'),
(3, 1, 'This is my test', '00:00:00', 0, 0, 0, 0, 'like'),
(4, 1, 'Hello', '838:59:59', 0, 0, 0, 0, 'like'),
(5, 1, 'hello', '00:00:00', 0, 0, 0, 0, 'like'),
(6, 1, 'Hello hi', '00:00:00', 0, 0, 0, 0, 'like'),
(7, 1, 'gmail', '00:00:00', 0, 0, 0, 0, 'like'),



--
-- Indexes for dumped tables
--

--
-- Indexes for table `feed_item`
--
ALTER TABLE `feed_item`
  ADD PRIMARY KEY (`ID`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `feed_item`
--
ALTER TABLE `feed_item`
  MODIFY `ID` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=188;
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 */;



-- phpMyAdmin SQL Dump
-- version 4.8.3
-- https://www.phpmyadmin.net/
--
-- Host: 127.0.0.1
-- Generation Time: Oct 09, 2018 at 04:20 PM
-- Server version: 10.1.36-MariaDB
-- PHP Version: 7.2.10

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET AUTOCOMMIT = 0;
START TRANSACTION;
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
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/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `gamersapi`
--

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

--
-- Table structure for table `user_friends`
--

CREATE TABLE `user_friends` (
  `ID` int(11) NOT NULL,
  `userid` int(11) NOT NULL,
  `friendid` int(11) NOT NULL,
  `status` int(11) NOT NULL,
  `timestamp` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

--
-- Dumping data for table `user_friends`
--

INSERT INTO `user_friends` (`ID`, `userid`, `friendid`, `status`, `timestamp`) VALUES
(1, 8, 4, 2, 1538369252),
(2, 1, 2, 2, 1538454842),
(3, 7, 1, 2, 1538455395),
(4, 7, 2, 2, 1538455487),
(5, 11, 2, 3, 1538455512),
(6, 6, 2, 2, 1538455567),
(7, 2, 5, 2, 1538456136),
(8, 1, 6, 1, 1538491568),
(9, 12, 1, 2, 1538499199),
(12, 1, 7, 1, 1538565860),
(13, 14, 1, 2, 1538800794);

--
-- Indexes for dumped tables
--

--
-- Indexes for table `user_friends`
--
ALTER TABLE `user_friends`
  ADD PRIMARY KEY (`ID`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `user_friends`
--
ALTER TABLE `user_friends`
  MODIFY `ID` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=14;

--
-- Constraints for dumped tables
--

--
-- Constraints for table `user_friends`
--
ALTER TABLE `user_friends`
  ADD CONSTRAINT `user_friends_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `users` (`ID`);
COMMIT;

/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
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Answer 1

考虑替换 ON 子句条件，AND ft.userid IN (SELECT u.ID ...，对于 WHERE 子句，因为此表达式设置在非 feed_item JOIN 上，特别是 users JOIN:

SELECT DISTINCT ft.ID as ID, ft.userid, ft.content, ft.timestamp, 
       ft.likes, ft.comments, ftc.comment, u.username, u.avatar 
FROM feed_item_comment ftc
LEFT JOIN feed_item ft 
  ON ftc.postid = ft.ID 
 AND ftc.userid = ft.userid
 AND ftc.userid != '".$this->user->info->ID."'
LEFT JOIN user_friends uf 
  ON uf.friendid = ftc.userid 
LEFT JOIN users u 
  ON u.ID = uf.friendid 
WHERE ft.userid IN               -- ONLY CHANGE
  (SELECT u.ID 
   FROM users u 
    WHERE u.ID IN (SELECT uf.friendid FROM user_friends uf 
                   WHERE uf.status = '2' AND uf.userid = '".$this->user->info->ID."') 
       OR u.ID IN (SELECT uf.userid FROM user_friends uf 
                   WHERE uf.status = '2' AND uf.friendid = '".$this->user->info->ID."')
       OR u.ID = '".$this->user->info->ID."'
  )     
ORDER BY ft.ID DESC

查看此 thread on WHERE vs ON 子句条件与 LEFT JOIN。