MySQL/Mariadb 问题：在 `Group by` 之前 `Order by DESC`
MySQL/Mariadb problem: `Order by DESC` before `Group by`
总结：我想在Group
之前Order by
我找到一篇关于这个问题的好文章，但最终没有成功修复它。
https://eddies-shop.medium.com/mysql-when-to-order-before-group-13d54d6c4ebb
我的服务器配置：
- 服务器类型：MariaDB
- 服务器版本：10.6.4-MariaDB - Arch Linux
关于查询： 我得到了房间和消息的列表，但我只需要每个房间的最新消息。
所以我需要按 conversation_id 分组，然后按 message_id or message_time.
排序
以上查询运行良好但不完整。
这样，对于每个房间，我们都有重复的行。
当我尝试取消注释查询的最后一行时
当我尝试申请 GROUP BY main.conversation_id 时。已经没有秩序了，秩序又被打破了
我的查询：
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
ORDER BY
sub.message_id DESC
) as main
# GROUP BY
# main.conversation_id
如果您需要了解更多关于数据库结构的信息：
--
-- Table structure for table `conversation`
--
CREATE TABLE `conversation` (
`id` int(50) NOT NULL,
`name` varchar(50) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`owner_id` int(50) NOT NULL,
`owner2_id` int(50) DEFAULT NULL,
`is_group` int(2) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`updated_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `conversation`
--
INSERT INTO `conversation` (`id`, `name`, `owner_id`, `owner2_id`, `is_group`, `created_at`, `updated_at`) VALUES
(7, 'تالار گفتگوی ریاضی', 1, NULL, 1, '2021-09-13 20:33:38', NULL),
(8, NULL, 2, 1, 0, '2021-09-13 20:33:46', '2021-09-14 07:55:44'),
(9, 'گروه ازمایشی', 3, NULL, 1, '2021-09-14 07:45:04', NULL),
(10, 'پروژه ها و ایده ها', 3, NULL, 1, '2021-09-14 07:47:19', NULL),
(11, NULL, 4, 1, 0, '2021-09-14 08:05:11', NULL);
--
-- Table structure for table `conversation_member`
--
CREATE TABLE `conversation_member` (
`id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`user_id` int(50) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `conversation_member`
--
INSERT INTO `conversation_member` (`id`, `conversation_id`, `user_id`, `created_at`) VALUES
(1, 8, 1, '2021-09-14 07:02:18'),
(2, 8, 2, '2021-09-14 07:02:18'),
(3, 7, 1, '2021-09-14 07:02:28'),
(4, 7, 3, '2021-09-14 07:02:28'),
(5, 9, 3, '2021-09-14 07:45:13'),
(7, 10, 4, '2021-09-14 08:02:57'),
(8, 10, 2, '2021-09-14 08:02:57'),
(9, 10, 1, '2021-09-14 08:03:05'),
(10, 11, 4, '2021-09-14 08:05:23'),
(11, 11, 1, '2021-09-14 08:05:23'),
(12, 7, 4, '2021-09-14 09:30:04');
--
-- Table structure for table `message`
--
CREATE TABLE `message` (
`id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`user_id` int(50) DEFAULT NULL,
`type` int(2) NOT NULL COMMENT '0=system,1=message,1=file,2=voice',
`body` text COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`filename` varchar(250) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`is_group` int(2) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`edited_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `message`
--
INSERT INTO `message` (`id`, `conversation_id`, `user_id`, `type`, `body`, `filename`, `is_group`, `created_at`, `edited_at`) VALUES
(1, 7, 1, 1, '1', NULL, 1, '2021-09-14 07:16:12', '2021-09-14 14:14:44'),
(2, 8, 1, 1, '2', NULL, 0, '2021-09-14 07:16:25', '2021-09-14 14:14:45'),
(3, 11, 1, 1, '3', NULL, 0, '2021-09-14 13:21:30', '2021-09-14 14:14:47'),
(4, 10, 1, 1, '4', NULL, 1, '2021-09-14 13:23:34', '2021-09-14 14:14:49'),
(5, 7, 1, 1, '5', NULL, 1, '2021-09-14 13:25:16', '2021-09-14 14:14:51'),
(6, 7, 1, 1, '6', NULL, 1, '2021-09-14 13:30:40', '2021-09-14 14:14:52'),
(7, 7, 1, 1, '7', NULL, 1, '2021-09-14 13:49:29', '2021-09-14 14:14:54'),
(8, 7, 1, 1, '8', NULL, 1, '2021-09-14 13:49:34', '2021-09-14 14:14:56'),
(9, 10, 1, 1, '9', NULL, 1, '2021-09-14 13:54:04', '2021-09-14 14:14:57'),
(10, 7, 1, 1, '10', NULL, 1, '2021-09-14 14:01:18', '2021-09-14 14:14:59'),
(11, 8, 1, 1, '11', NULL, 0, '2021-09-14 14:07:48', '2021-09-14 14:15:03'),
(12, 11, 4, 1, 'test-new', NULL, 0, '2021-09-14 15:11:51', NULL),
(13, 11, 1, 1, 'fdgdfg', NULL, 0, '2021-09-14 13:21:30', '2021-09-14 14:14:47'),
(14, 11, 1, 1, 'sd1f23sd1f', NULL, 0, '2021-09-14 15:33:02', NULL),
(15, 11, 1, 1, 'dfgdfgdfgdfg', NULL, 0, '2021-09-14 15:33:02', NULL),
(16, 11, 1, 1, 'dfgdfgdfg', NULL, 0, '2021-09-14 15:33:06', NULL),
(17, 11, 1, 1, 'dfg345345345', NULL, 0, '2021-09-14 15:33:06', NULL),
(18, 11, 1, 1, 'gdfg234234234', NULL, 0, '2021-09-14 15:33:17', NULL),
(19, 11, 1, 1, 'dfgda1323123f', NULL, 0, '2021-09-14 15:33:17', NULL),
(20, 11, 1, 1, '234234234234', NULL, 0, '2021-09-14 15:33:17', NULL),
(21, 11, 1, 1, '345345345345', NULL, 0, '2021-09-14 15:33:17', NULL),
(22, 11, 1, 1, '5565656', NULL, 0, '2021-09-14 15:33:17', NULL),
(23, 11, 1, 1, '7787878', NULL, 0, '2021-09-14 15:33:17', NULL),
(24, 11, 1, 1, 'یبلیبلیبلیبل', NULL, 0, '2021-09-14 15:33:28', NULL),
(25, 11, 1, 1, 'ض۳۲ث۱۲۳۴۲۳۴۲۳۴', NULL, 0, '2021-09-14 15:33:28', NULL),
(26, 11, 1, 1, '۳۴۵سیبیلبیبلب', NULL, 0, '2021-09-14 15:33:28', NULL),
(27, 11, 1, 1, 'فقفثفثقفثقف', NULL, 0, '2021-09-14 15:33:28', NULL),
(28, 11, 1, 1, '۳۳۴۲۴۲۳۴', NULL, 0, '2021-09-14 15:33:28', NULL),
(29, 11, 1, 1, '$$$$', NULL, 0, '2021-09-14 15:33:28', NULL),
(30, 11, 1, 1, '$$$%%dfgdfg', NULL, 0, '2021-09-14 15:33:47', NULL),
(31, 11, 1, 1, 'dfgdfg23423423423سیبلیبل', NULL, 0, '2021-09-14 15:33:47', NULL),
(32, 11, 1, 1, 'یبلص۴۳۵۲۳۴۵۳۴۵۳۴۵', NULL, 0, '2021-09-14 15:33:47', NULL),
(33, 11, 1, 1, 'یبل۳۵۳۴۵فثقیبلیبلیبل', NULL, 0, '2021-09-14 15:33:47', NULL),
(34, 11, 1, 1, 'یبلیلبل۳۴۵۳۴۵۳۴۵۳۴۵۳۴۵', NULL, 0, '2021-09-14 15:33:47', NULL),
(35, 11, 1, 1, '$$$$$####', NULL, 0, '2021-09-14 15:33:47', NULL);
--
-- Table structure for table `message_view`
--
CREATE TABLE `message_view` (
`id` int(50) NOT NULL,
`message_id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`is_group` int(2) NOT NULL,
`user_id` int(50) NOT NULL,
`viewed_at` timestamp NOT NULL DEFAULT current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Table structure for table `session`
--
CREATE TABLE `session` (
`id` int(50) NOT NULL,
`user_id` int(50) NOT NULL,
`device` varchar(250) COLLATE utf8mb4_unicode_ci NOT NULL,
`code` int(10) DEFAULT NULL,
`secret` varchar(250) COLLATE utf8mb4_unicode_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `session`
--
INSERT INTO `session` (`id`, `user_id`, `device`, `code`, `secret`) VALUES
(1, 1, '08ad5559-15f7-4c32-ab2d-2d8a563670a3', 0, '1'),
(2, 1, 'c8216675-08ff-4deb-9341-2afbe88bc716', NULL, '41234'),
(3, 1, 'f20279f4-615d-4b6f-83e4-0e6c201395e3', 53741, NULL),
(4, 1, 'bef6f3e2-04b7-4ff5-b948-c035c376f4d2', 51003, NULL),
(5, 1, '3492d860-6ae4-4a00-a65d-346d880c4e71', 30646, NULL),
(6, 1, 'fd0b2d35-d7aa-4e40-b362-6b592dc17aad', 28576, NULL),
(7, 1, 'c69516b4-53a8-4e69-a874-a11d859b451d', 32440, NULL),
(8, 1, '075149a4-d94c-4246-99ce-d0b71c72f26c', 99800, NULL),
(9, 1, '4804b854-89b6-4c85-8df3-815ffdd34fba', 19774, NULL),
(10, 1, 'a60195ac-2e50-42ed-9d71-1d3d04729339', 10262, NULL),
(11, 1, '80a9a2e7-ee8b-47ef-8ca4-75216721a6ac', 88424, 'bc82131e-0e59-4841-98b7-798cf65d9fcb'),
(12, 1, '6c120179-312f-4d73-9488-7f692cb54234', 42832, NULL),
(13, 1, 'a8b7ca5b-a47d-48b0-afeb-197b5ec7dc44', 39034, 'fdd51fe1-5bae-424f-9515-1a1b435faed6'),
(14, 1, '0652aa2a-01df-497d-b7b2-77fc87a29c24', 36133, '8fc206e1-1a49-444b-bcb6-ea289c17a918'),
(15, 1, '6b0bf6b9-4d32-4c6b-9e84-fd0b393acb31', 87972, '05de4b7a-cc6e-4ff0-b321-72473ac903bd'),
(16, 1, '0a523464-89ab-4f4a-803e-ca252e637e4f', 40843, 'a35a2169-949d-4043-a061-1a56ec30440e'),
(17, 1, '34b2e1bf-a088-48e8-85b8-1943db001fd5', 65916, '971a3cea-8ccc-45bf-887e-7797e4c6ab22'),
(18, 1, 'fcdb6c98-d044-43e8-a373-351f7ca1536d', 82257, 'bacb442d-e066-4117-a380-c468316d47f2'),
(19, 1, '565a036d-c4f8-46b0-8493-e5371e3dccca', 71626, '610b4e81-cf72-4091-a711-d64c601e0f0c'),
(20, 1, '8775a2d9-4544-48c4-ab5c-6d7216c955f7', 46912, 'b5eabb83-e372-4e7b-a48a-3c99eaba5d6d'),
(21, 1, 'a94f734b-0aa0-4ee9-aea3-86e2a405cf56', 44508, '2db51630-e32a-4172-90aa-99d5f7b00063'),
(22, 1, 'b0ed316b-483b-47cc-b27f-2fe6b83f410a', 87850, 'f68377a4-3ed8-4c36-8a78-1d807ed50449'),
(23, 1, '08f38458-cd12-4a9c-9c63-9c6aa291956b', 79266, 'e6314070-8401-4d7b-b9d7-a44c147c75ec'),
(24, 4, '192dfd3f-b2d7-401f-bf95-2d663b6badab', 65088, '6fa66aa9-f47d-46fd-9ce5-9645802383da'),
(25, 4, 'fc8fe94b-f220-42db-ab77-74de994f8275', 37715, '0e188502-67c2-44a9-b68c-32b07fa150ab'),
(26, 1, '27451de6-c730-450a-b76c-3ea53ff74580', 16934, '662ae741-fa42-4fc3-b0bb-5ae6c9e67b52'),
(27, 4, '6ba92f55-cffe-4d9e-b646-be9cf07e99e3', 91683, '27b85d09-831d-41cd-8032-17743a76616d');
--
-- Table structure for table `user`
--
CREATE TABLE `user` (
`id` int(50) NOT NULL,
`first_name` varchar(40) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`last_name` varchar(40) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`country_code` int(2) NOT NULL,
`phone_number` varchar(13) COLLATE utf8mb4_unicode_ci NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`updated_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `user`
--
INSERT INTO `user` (`id`, `first_name`, `last_name`, `country_code`, `phone_number`, `created_at`, `updated_at`) VALUES
(1, 'Max', 'Base', 98, '9134458080', '2021-09-13 18:28:47', '2021-09-14 07:13:21'),
(2, 'Ali', 'Tahmasebi', 98, '91032545254', '2021-09-14 07:02:46', '2021-09-14 08:02:24'),
(3, 'B.', 'KheirKhah', 98, '9124554020', '2021-09-14 07:02:46', '2021-09-14 08:03:20'),
(4, 'H.', 'Malekian', 98, '9134550773', '2021-09-14 07:25:21', '2021-09-14 08:02:35');
--
-- Indexes for dumped tables
--
--
-- Indexes for table `conversation`
--
ALTER TABLE `conversation`
ADD PRIMARY KEY (`id`),
ADD KEY `owner_id` (`owner_id`),
ADD KEY `is_group` (`is_group`);
--
-- Indexes for table `conversation_member`
--
ALTER TABLE `conversation_member`
ADD PRIMARY KEY (`id`);
--
-- Indexes for table `message`
--
ALTER TABLE `message`
ADD PRIMARY KEY (`id`),
ADD KEY `is_group` (`is_group`),
ADD KEY `user_id` (`user_id`),
ADD KEY `conversation_id` (`conversation_id`);
--
-- Indexes for table `message_view`
--
ALTER TABLE `message_view`
ADD PRIMARY KEY (`id`);
--
-- Indexes for table `session`
--
ALTER TABLE `session`
ADD PRIMARY KEY (`id`),
ADD KEY `user_id` (`user_id`),
ADD KEY `device` (`device`);
--
-- Indexes for table `user`
--
ALTER TABLE `user`
ADD PRIMARY KEY (`id`),
ADD KEY `phone_number` (`phone_number`),
ADD KEY `country_code` (`country_code`);
--
-- AUTO_INCREMENT for dumped tables
--
--
-- AUTO_INCREMENT for table `conversation`
--
ALTER TABLE `conversation`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=12;
--
-- AUTO_INCREMENT for table `conversation_member`
--
ALTER TABLE `conversation_member`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=13;
--
-- AUTO_INCREMENT for table `message`
--
ALTER TABLE `message`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=12;
--
-- AUTO_INCREMENT for table `message_view`
--
ALTER TABLE `message_view`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT;
--
-- AUTO_INCREMENT for table `session`
--
ALTER TABLE `session`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=28;
--
-- AUTO_INCREMENT for table `user`
--
ALTER TABLE `user`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=5;
COMMIT;
关于数据库和table的更多信息：
conversation、conversation_membertable的目的：我有一个数据库来存储信使数据。有 2 个模型对话：
- 1：组：多人
- 2：个人聊天：一个用户与另一个用户（这就是为什么我有
owner2_id 列。）
主要查询是合并 ID = 1:
用户的个人聊天和群组聊天
(
SELECT
sub1.*
FROM
(
SELECT
conversation.id AS conversation_id,
conversation.name AS conversation_name,
conversation.is_group AS conversation_isgroup,
conversation.owner_id AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
WHERE
conversation_member.user_id = 1
AND
conversation.is_group = 1
) AS sub1
GROUP BY sub1.message_id desc
)
UNION
(
SELECT
sub2.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub2
GROUP BY sub2.message_id desc
);
我在做什么
我想要一个所有群组对话和个人对话的列表以及该房间中的最后消息，并按最后 MESSAGE_TIME 对所有房间进行排序。
几乎所有信使都会遇到这种情况。
我不是SQL方面的高手，顺便说一句，我只是在几个小时后成功解决了一个查询的问题！
如果您对此有任何更好的疑问，请post并回答。
这只是针对一个查询，我有两个查询需要联合合并。
单个查询：
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
GROUP BY sub.message_id desc
) as main
GROUP BY main.conversation_id
主查询和UNION合并结果：
(
SELECT
mm.*
FROM
(
(
SELECT
sub1.*
FROM
(
SELECT
conversation.id AS conversation_id,
conversation.name AS conversation_name,
conversation.is_group AS conversation_isgroup,
conversation.owner_id AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
WHERE
conversation_member.user_id = 1
AND
conversation.is_group = 1
) AS sub1
GROUP BY sub1.message_id desc
)
UNION
(
SELECT
sub2.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub2
GROUP BY sub2.message_id desc
)
) AS mm
GROUP BY mm.conversation_id desc
)
;
我不确定查询是否优化或良好。
但这有效。
如果这还不够好，请查看并评论我的查询。
我只会用你问题的查询来展示MariaDB如何处理子查询中的顺序。
我将你的 user table 更改为 user01 因为我的 MariaDB 数据库中有一个 table user。
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user01.first_name, " ", user01.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user01
ON
user01.id = message.user_id
LEFT JOIN
user01 as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
ORDER BY
sub.message_id DESC limit 10
) as main
GROUP BY
main.conversation_id;
我只在 sub.message_id DESC limit 10 上添加了 limit 10。
A "table" (and subquery in the FROM clause too) is - according to the
SQL standard - an unordered set of rows. Rows in a table (or in a
subquery in the FROM clause) do not come in any specific order. That's
why the optimizer can ignore the ORDER BY clause that you have
specified. In fact, the SQL standard does not even allow the ORDER BY
clause to appear in this subquery (we allow it, because ORDER BY ...
LIMIT ... changes the result, the set of rows, not only their order).
You need to treat the subquery in the FROM clause, as a set of rows in
some unspecified and undefined order, and put the ORDER BY on the
top-level SELECT
https://mariadb.com/kb/en/why-is-order-by-in-a-from-subquery-ignored/
总结：我想在Group
Order by
我找到一篇关于这个问题的好文章，但最终没有成功修复它。 https://eddies-shop.medium.com/mysql-when-to-order-before-group-13d54d6c4ebb
我的服务器配置：
- 服务器类型：MariaDB
- 服务器版本：10.6.4-MariaDB - Arch Linux
关于查询： 我得到了房间和消息的列表，但我只需要每个房间的最新消息。
所以我需要按 conversation_id 分组，然后按 message_id or message_time.
以上查询运行良好但不完整。 这样，对于每个房间，我们都有重复的行。
当我尝试取消注释查询的最后一行时
当我尝试申请 GROUP BY main.conversation_id 时。已经没有秩序了，秩序又被打破了
我的查询：
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
ORDER BY
sub.message_id DESC
) as main
# GROUP BY
# main.conversation_id
如果您需要了解更多关于数据库结构的信息：
--
-- Table structure for table `conversation`
--
CREATE TABLE `conversation` (
`id` int(50) NOT NULL,
`name` varchar(50) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`owner_id` int(50) NOT NULL,
`owner2_id` int(50) DEFAULT NULL,
`is_group` int(2) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`updated_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `conversation`
--
INSERT INTO `conversation` (`id`, `name`, `owner_id`, `owner2_id`, `is_group`, `created_at`, `updated_at`) VALUES
(7, 'تالار گفتگوی ریاضی', 1, NULL, 1, '2021-09-13 20:33:38', NULL),
(8, NULL, 2, 1, 0, '2021-09-13 20:33:46', '2021-09-14 07:55:44'),
(9, 'گروه ازمایشی', 3, NULL, 1, '2021-09-14 07:45:04', NULL),
(10, 'پروژه ها و ایده ها', 3, NULL, 1, '2021-09-14 07:47:19', NULL),
(11, NULL, 4, 1, 0, '2021-09-14 08:05:11', NULL);
--
-- Table structure for table `conversation_member`
--
CREATE TABLE `conversation_member` (
`id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`user_id` int(50) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `conversation_member`
--
INSERT INTO `conversation_member` (`id`, `conversation_id`, `user_id`, `created_at`) VALUES
(1, 8, 1, '2021-09-14 07:02:18'),
(2, 8, 2, '2021-09-14 07:02:18'),
(3, 7, 1, '2021-09-14 07:02:28'),
(4, 7, 3, '2021-09-14 07:02:28'),
(5, 9, 3, '2021-09-14 07:45:13'),
(7, 10, 4, '2021-09-14 08:02:57'),
(8, 10, 2, '2021-09-14 08:02:57'),
(9, 10, 1, '2021-09-14 08:03:05'),
(10, 11, 4, '2021-09-14 08:05:23'),
(11, 11, 1, '2021-09-14 08:05:23'),
(12, 7, 4, '2021-09-14 09:30:04');
--
-- Table structure for table `message`
--
CREATE TABLE `message` (
`id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`user_id` int(50) DEFAULT NULL,
`type` int(2) NOT NULL COMMENT '0=system,1=message,1=file,2=voice',
`body` text COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`filename` varchar(250) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`is_group` int(2) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`edited_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `message`
--
INSERT INTO `message` (`id`, `conversation_id`, `user_id`, `type`, `body`, `filename`, `is_group`, `created_at`, `edited_at`) VALUES
(1, 7, 1, 1, '1', NULL, 1, '2021-09-14 07:16:12', '2021-09-14 14:14:44'),
(2, 8, 1, 1, '2', NULL, 0, '2021-09-14 07:16:25', '2021-09-14 14:14:45'),
(3, 11, 1, 1, '3', NULL, 0, '2021-09-14 13:21:30', '2021-09-14 14:14:47'),
(4, 10, 1, 1, '4', NULL, 1, '2021-09-14 13:23:34', '2021-09-14 14:14:49'),
(5, 7, 1, 1, '5', NULL, 1, '2021-09-14 13:25:16', '2021-09-14 14:14:51'),
(6, 7, 1, 1, '6', NULL, 1, '2021-09-14 13:30:40', '2021-09-14 14:14:52'),
(7, 7, 1, 1, '7', NULL, 1, '2021-09-14 13:49:29', '2021-09-14 14:14:54'),
(8, 7, 1, 1, '8', NULL, 1, '2021-09-14 13:49:34', '2021-09-14 14:14:56'),
(9, 10, 1, 1, '9', NULL, 1, '2021-09-14 13:54:04', '2021-09-14 14:14:57'),
(10, 7, 1, 1, '10', NULL, 1, '2021-09-14 14:01:18', '2021-09-14 14:14:59'),
(11, 8, 1, 1, '11', NULL, 0, '2021-09-14 14:07:48', '2021-09-14 14:15:03'),
(12, 11, 4, 1, 'test-new', NULL, 0, '2021-09-14 15:11:51', NULL),
(13, 11, 1, 1, 'fdgdfg', NULL, 0, '2021-09-14 13:21:30', '2021-09-14 14:14:47'),
(14, 11, 1, 1, 'sd1f23sd1f', NULL, 0, '2021-09-14 15:33:02', NULL),
(15, 11, 1, 1, 'dfgdfgdfgdfg', NULL, 0, '2021-09-14 15:33:02', NULL),
(16, 11, 1, 1, 'dfgdfgdfg', NULL, 0, '2021-09-14 15:33:06', NULL),
(17, 11, 1, 1, 'dfg345345345', NULL, 0, '2021-09-14 15:33:06', NULL),
(18, 11, 1, 1, 'gdfg234234234', NULL, 0, '2021-09-14 15:33:17', NULL),
(19, 11, 1, 1, 'dfgda1323123f', NULL, 0, '2021-09-14 15:33:17', NULL),
(20, 11, 1, 1, '234234234234', NULL, 0, '2021-09-14 15:33:17', NULL),
(21, 11, 1, 1, '345345345345', NULL, 0, '2021-09-14 15:33:17', NULL),
(22, 11, 1, 1, '5565656', NULL, 0, '2021-09-14 15:33:17', NULL),
(23, 11, 1, 1, '7787878', NULL, 0, '2021-09-14 15:33:17', NULL),
(24, 11, 1, 1, 'یبلیبلیبلیبل', NULL, 0, '2021-09-14 15:33:28', NULL),
(25, 11, 1, 1, 'ض۳۲ث۱۲۳۴۲۳۴۲۳۴', NULL, 0, '2021-09-14 15:33:28', NULL),
(26, 11, 1, 1, '۳۴۵سیبیلبیبلب', NULL, 0, '2021-09-14 15:33:28', NULL),
(27, 11, 1, 1, 'فقفثفثقفثقف', NULL, 0, '2021-09-14 15:33:28', NULL),
(28, 11, 1, 1, '۳۳۴۲۴۲۳۴', NULL, 0, '2021-09-14 15:33:28', NULL),
(29, 11, 1, 1, '$$$$', NULL, 0, '2021-09-14 15:33:28', NULL),
(30, 11, 1, 1, '$$$%%dfgdfg', NULL, 0, '2021-09-14 15:33:47', NULL),
(31, 11, 1, 1, 'dfgdfg23423423423سیبلیبل', NULL, 0, '2021-09-14 15:33:47', NULL),
(32, 11, 1, 1, 'یبلص۴۳۵۲۳۴۵۳۴۵۳۴۵', NULL, 0, '2021-09-14 15:33:47', NULL),
(33, 11, 1, 1, 'یبل۳۵۳۴۵فثقیبلیبلیبل', NULL, 0, '2021-09-14 15:33:47', NULL),
(34, 11, 1, 1, 'یبلیلبل۳۴۵۳۴۵۳۴۵۳۴۵۳۴۵', NULL, 0, '2021-09-14 15:33:47', NULL),
(35, 11, 1, 1, '$$$$$####', NULL, 0, '2021-09-14 15:33:47', NULL);
--
-- Table structure for table `message_view`
--
CREATE TABLE `message_view` (
`id` int(50) NOT NULL,
`message_id` int(50) NOT NULL,
`conversation_id` int(50) NOT NULL,
`is_group` int(2) NOT NULL,
`user_id` int(50) NOT NULL,
`viewed_at` timestamp NOT NULL DEFAULT current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Table structure for table `session`
--
CREATE TABLE `session` (
`id` int(50) NOT NULL,
`user_id` int(50) NOT NULL,
`device` varchar(250) COLLATE utf8mb4_unicode_ci NOT NULL,
`code` int(10) DEFAULT NULL,
`secret` varchar(250) COLLATE utf8mb4_unicode_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `session`
--
INSERT INTO `session` (`id`, `user_id`, `device`, `code`, `secret`) VALUES
(1, 1, '08ad5559-15f7-4c32-ab2d-2d8a563670a3', 0, '1'),
(2, 1, 'c8216675-08ff-4deb-9341-2afbe88bc716', NULL, '41234'),
(3, 1, 'f20279f4-615d-4b6f-83e4-0e6c201395e3', 53741, NULL),
(4, 1, 'bef6f3e2-04b7-4ff5-b948-c035c376f4d2', 51003, NULL),
(5, 1, '3492d860-6ae4-4a00-a65d-346d880c4e71', 30646, NULL),
(6, 1, 'fd0b2d35-d7aa-4e40-b362-6b592dc17aad', 28576, NULL),
(7, 1, 'c69516b4-53a8-4e69-a874-a11d859b451d', 32440, NULL),
(8, 1, '075149a4-d94c-4246-99ce-d0b71c72f26c', 99800, NULL),
(9, 1, '4804b854-89b6-4c85-8df3-815ffdd34fba', 19774, NULL),
(10, 1, 'a60195ac-2e50-42ed-9d71-1d3d04729339', 10262, NULL),
(11, 1, '80a9a2e7-ee8b-47ef-8ca4-75216721a6ac', 88424, 'bc82131e-0e59-4841-98b7-798cf65d9fcb'),
(12, 1, '6c120179-312f-4d73-9488-7f692cb54234', 42832, NULL),
(13, 1, 'a8b7ca5b-a47d-48b0-afeb-197b5ec7dc44', 39034, 'fdd51fe1-5bae-424f-9515-1a1b435faed6'),
(14, 1, '0652aa2a-01df-497d-b7b2-77fc87a29c24', 36133, '8fc206e1-1a49-444b-bcb6-ea289c17a918'),
(15, 1, '6b0bf6b9-4d32-4c6b-9e84-fd0b393acb31', 87972, '05de4b7a-cc6e-4ff0-b321-72473ac903bd'),
(16, 1, '0a523464-89ab-4f4a-803e-ca252e637e4f', 40843, 'a35a2169-949d-4043-a061-1a56ec30440e'),
(17, 1, '34b2e1bf-a088-48e8-85b8-1943db001fd5', 65916, '971a3cea-8ccc-45bf-887e-7797e4c6ab22'),
(18, 1, 'fcdb6c98-d044-43e8-a373-351f7ca1536d', 82257, 'bacb442d-e066-4117-a380-c468316d47f2'),
(19, 1, '565a036d-c4f8-46b0-8493-e5371e3dccca', 71626, '610b4e81-cf72-4091-a711-d64c601e0f0c'),
(20, 1, '8775a2d9-4544-48c4-ab5c-6d7216c955f7', 46912, 'b5eabb83-e372-4e7b-a48a-3c99eaba5d6d'),
(21, 1, 'a94f734b-0aa0-4ee9-aea3-86e2a405cf56', 44508, '2db51630-e32a-4172-90aa-99d5f7b00063'),
(22, 1, 'b0ed316b-483b-47cc-b27f-2fe6b83f410a', 87850, 'f68377a4-3ed8-4c36-8a78-1d807ed50449'),
(23, 1, '08f38458-cd12-4a9c-9c63-9c6aa291956b', 79266, 'e6314070-8401-4d7b-b9d7-a44c147c75ec'),
(24, 4, '192dfd3f-b2d7-401f-bf95-2d663b6badab', 65088, '6fa66aa9-f47d-46fd-9ce5-9645802383da'),
(25, 4, 'fc8fe94b-f220-42db-ab77-74de994f8275', 37715, '0e188502-67c2-44a9-b68c-32b07fa150ab'),
(26, 1, '27451de6-c730-450a-b76c-3ea53ff74580', 16934, '662ae741-fa42-4fc3-b0bb-5ae6c9e67b52'),
(27, 4, '6ba92f55-cffe-4d9e-b646-be9cf07e99e3', 91683, '27b85d09-831d-41cd-8032-17743a76616d');
--
-- Table structure for table `user`
--
CREATE TABLE `user` (
`id` int(50) NOT NULL,
`first_name` varchar(40) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`last_name` varchar(40) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
`country_code` int(2) NOT NULL,
`phone_number` varchar(13) COLLATE utf8mb4_unicode_ci NOT NULL,
`created_at` timestamp NOT NULL DEFAULT current_timestamp(),
`updated_at` timestamp NULL DEFAULT NULL ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
--
-- Dumping data for table `user`
--
INSERT INTO `user` (`id`, `first_name`, `last_name`, `country_code`, `phone_number`, `created_at`, `updated_at`) VALUES
(1, 'Max', 'Base', 98, '9134458080', '2021-09-13 18:28:47', '2021-09-14 07:13:21'),
(2, 'Ali', 'Tahmasebi', 98, '91032545254', '2021-09-14 07:02:46', '2021-09-14 08:02:24'),
(3, 'B.', 'KheirKhah', 98, '9124554020', '2021-09-14 07:02:46', '2021-09-14 08:03:20'),
(4, 'H.', 'Malekian', 98, '9134550773', '2021-09-14 07:25:21', '2021-09-14 08:02:35');
--
-- Indexes for dumped tables
--
--
-- Indexes for table `conversation`
--
ALTER TABLE `conversation`
ADD PRIMARY KEY (`id`),
ADD KEY `owner_id` (`owner_id`),
ADD KEY `is_group` (`is_group`);
--
-- Indexes for table `conversation_member`
--
ALTER TABLE `conversation_member`
ADD PRIMARY KEY (`id`);
--
-- Indexes for table `message`
--
ALTER TABLE `message`
ADD PRIMARY KEY (`id`),
ADD KEY `is_group` (`is_group`),
ADD KEY `user_id` (`user_id`),
ADD KEY `conversation_id` (`conversation_id`);
--
-- Indexes for table `message_view`
--
ALTER TABLE `message_view`
ADD PRIMARY KEY (`id`);
--
-- Indexes for table `session`
--
ALTER TABLE `session`
ADD PRIMARY KEY (`id`),
ADD KEY `user_id` (`user_id`),
ADD KEY `device` (`device`);
--
-- Indexes for table `user`
--
ALTER TABLE `user`
ADD PRIMARY KEY (`id`),
ADD KEY `phone_number` (`phone_number`),
ADD KEY `country_code` (`country_code`);
--
-- AUTO_INCREMENT for dumped tables
--
--
-- AUTO_INCREMENT for table `conversation`
--
ALTER TABLE `conversation`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=12;
--
-- AUTO_INCREMENT for table `conversation_member`
--
ALTER TABLE `conversation_member`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=13;
--
-- AUTO_INCREMENT for table `message`
--
ALTER TABLE `message`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=12;
--
-- AUTO_INCREMENT for table `message_view`
--
ALTER TABLE `message_view`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT;
--
-- AUTO_INCREMENT for table `session`
--
ALTER TABLE `session`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=28;
--
-- AUTO_INCREMENT for table `user`
--
ALTER TABLE `user`
MODIFY `id` int(50) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=5;
COMMIT;
关于数据库和table的更多信息：
conversation、conversation_membertable的目的：我有一个数据库来存储信使数据。有 2 个模型对话：
- 1：组：多人
- 2：个人聊天：一个用户与另一个用户（这就是为什么我有
owner2_id列。）
主要查询是合并 ID = 1:
(
SELECT
sub1.*
FROM
(
SELECT
conversation.id AS conversation_id,
conversation.name AS conversation_name,
conversation.is_group AS conversation_isgroup,
conversation.owner_id AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
WHERE
conversation_member.user_id = 1
AND
conversation.is_group = 1
) AS sub1
GROUP BY sub1.message_id desc
)
UNION
(
SELECT
sub2.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub2
GROUP BY sub2.message_id desc
);
我在做什么
我想要一个所有群组对话和个人对话的列表以及该房间中的最后消息，并按最后 MESSAGE_TIME 对所有房间进行排序。
几乎所有信使都会遇到这种情况。
我不是SQL方面的高手，顺便说一句，我只是在几个小时后成功解决了一个查询的问题！
如果您对此有任何更好的疑问，请post并回答。
这只是针对一个查询，我有两个查询需要联合合并。
单个查询：
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
GROUP BY sub.message_id desc
) as main
GROUP BY main.conversation_id
主查询和UNION合并结果：
(
SELECT
mm.*
FROM
(
(
SELECT
sub1.*
FROM
(
SELECT
conversation.id AS conversation_id,
conversation.name AS conversation_name,
conversation.is_group AS conversation_isgroup,
conversation.owner_id AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
WHERE
conversation_member.user_id = 1
AND
conversation.is_group = 1
) AS sub1
GROUP BY sub1.message_id desc
)
UNION
(
SELECT
sub2.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user.first_name, " ", user.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user
ON
user.id = message.user_id
LEFT JOIN
user as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub2
GROUP BY sub2.message_id desc
)
) AS mm
GROUP BY mm.conversation_id desc
)
;
我不确定查询是否优化或良好。 但这有效。 如果这还不够好，请查看并评论我的查询。
我只会用你问题的查询来展示MariaDB如何处理子查询中的顺序。
我将你的 user table 更改为 user01 因为我的 MariaDB 数据库中有一个 table user。
SELECT
main.*
FROM
(
SELECT
sub.*
FROM
(
SELECT
conversation.id AS conversation_id,
CONCAT(user2.first_name, " ", user2.last_name) AS conversation_name,
conversation.is_group AS conversation_isgroup,
(SELECT user_id FROM conversation_member WHERE conversation_id = conversation.id AND user_id != 1) AS conversation_owner_id,
message.id AS message_id,
message.type AS message_type,
message.body AS message_body,
message.filename AS message_filename,
message.created_at AS message_time,
message.user_id AS message_user_id,
CONCAT(user01.first_name, " ", user01.last_name) AS message_user_name
FROM
conversation
INNER JOIN
conversation_member
ON
conversation_member.conversation_id = conversation.id
LEFT JOIN
message
ON
message.conversation_id = conversation.id
LEFT JOIN
user01
ON
user01.id = message.user_id
LEFT JOIN
user01 as user2
ON
user2.id = conversation.owner2_id
OR
user2.id = conversation.owner_id
WHERE
user2.id != 1
AND
name IS NULL
AND
conversation_member.user_id = 1
AND
conversation.is_group = 0
) AS sub
ORDER BY
sub.message_id DESC limit 10
) as main
GROUP BY
main.conversation_id;
我只在 sub.message_id DESC limit 10 上添加了 limit 10。
A "table" (and subquery in the FROM clause too) is - according to the SQL standard - an unordered set of rows. Rows in a table (or in a subquery in the FROM clause) do not come in any specific order. That's why the optimizer can ignore the ORDER BY clause that you have specified. In fact, the SQL standard does not even allow the ORDER BY clause to appear in this subquery (we allow it, because ORDER BY ... LIMIT ... changes the result, the set of rows, not only their order).
You need to treat the subquery in the FROM clause, as a set of rows in some unspecified and undefined order, and put the ORDER BY on the top-level SELECT
https://mariadb.com/kb/en/why-is-order-by-in-a-from-subquery-ignored/