Question

如何从回复和帖子中获取最后5条帖子（按日期（created_at））。

示例胜于解释：

StateEndTime

预期结果是：

-- topics --
id  -  title  -  content  -  created_at
----------------------------------------
 1  -  title  -  content  -  2019-02-05 19:14:41
 2  -  title2 -  content2 -  2019-02-05 19:14:42
 3  -  title3 -  content3 -  2019-02-05 19:14:43
 4  -  title4 -  content4 -  2019-02-05 19:14:44
 5  -  title5 -  content5 -  2019-02-05 19:14:45

-- replies --
id  -  content  -  created_at
--------------------------
 1  -  content  -  2019-02-05 19:14:41
 2  -  content2 -  2019-02-05 19:14:42
 3  -  content3 -  2019-02-05 19:14:43
 4  -  content4 -  2019-02-05 19:14:44
 5  -  content5 -  2019-02-05 19:14:45

我正在使用MySQL

Answer 1

这应该回答您的问题

模式（MySQL v5.7）

CREATE TABLE topics (
  `id` INTEGER,
  `title` VARCHAR(6),
  `content` VARCHAR(8),
  `created_at` datetime
);

INSERT INTO topics
  (`id`, `title`, `content`, `created_at`)
VALUES
  ('1', 'title', 'content', '2019-02-05 19:14:41'),
  ('2', 'title2', 'content2', '2019-02-05 19:14:42'),
  ('3', 'title3', 'content3', '2019-02-05 19:14:43'),
  ('4', 'title4', 'content4', '2019-02-05 19:14:44'),
  ('5', 'title5', 'content5', '2019-02-05 19:14:45');

CREATE TABLE replies (
  `id` INTEGER,
  `content` VARCHAR(8),
  `created_at` datetime
);

INSERT INTO replies
  (`id`, `content`, `created_at`)
VALUES
  ('1', 'content', '2019-02-05 19:14:41'),
  ('2', 'content2', '2019-02-05 19:14:42'),
  ('3', 'content3', '2019-02-05 19:14:43'),
  ('4', 'content4', '2019-02-05 19:14:44'),
  ('5', 'content5', '2019-02-05 19:14:45');

查询＃1

select * from
(
 select * from topics
 union all 
 select id, null, content, created_at from replies
) t 
ORDER BY created_at desc
LIMIT 5;

结果

| id  | title  | content  | created_at          |
| --- | ------ | -------- | ------------------- |
| 5   |        | content5 | 2019-02-05 19:14:45 |
| 5   | title5 | content5 | 2019-02-05 19:14:45 |
| 4   |        | content4 | 2019-02-05 19:14:44 |
| 4   | title4 | content4 | 2019-02-05 19:14:44 |
| 3   |        | content3 | 2019-02-05 19:14:43 |

View on DB Fiddle

Answer 2

您可以合并两个获取所有信息的查询，按日期排序，然后限制为5个结果

模式（MySQL v5.7）

CREATE TABLE topics (
  `id` INTEGER,
  `title` VARCHAR(6),
  `content` VARCHAR(8),
  `created_at` DATETIME
);

INSERT INTO topics
  (`id`, `title`, `content`, `created_at`)
VALUES
  (1, 'title', 'content', '2019-02-05 19:14:41'),
  (2, 'title2', 'content2', '2019-02-05 19:14:42'),
  (3, 'title3', 'content3', '2019-02-05 19:14:43'),
  (4, 'title4', 'content4', '2019-02-05 19:14:44'),
  (5, 'title5', 'content5', '2019-02-05 19:14:45');

CREATE TABLE replies (
  `id` INTEGER,
  `content` VARCHAR(8),
  `created_at` DATETIME
);

INSERT INTO replies
  (`id`, `content`, `created_at`)
VALUES
  (1, 'content', '2019-02-05 19:14:41'),
  (2, 'content2', '2019-02-05 19:14:42'),
  (3, 'content3', '2019-02-05 19:14:43'),
  (4, 'content4', '2019-02-05 19:14:44'),
  (5, 'content5', '2019-02-05 19:14:45');

查询＃1

SELECT id, CONCAT(title, ' - ', content) AS "title - content", created_at
FROM topics
UNION ALL
SELECT id, content, created_at
FROM replies
ORDER BY created_at DESC LIMIT 5;

输出

| id  | title - content   | created_at          |
| --- | ----------------- | ------------------- |
| 5   | title5 - content5 | 2019-02-05 19:14:45 |
| 5   | content5          | 2019-02-05 19:14:45 |
| 4   | content4          | 2019-02-05 19:14:44 |
| 4   | title4 - content4 | 2019-02-05 19:14:44 |
| 3   | content3          | 2019-02-05 19:14:43 |

查询＃2

-- If you want to sort them by date ascending, you can nest the previous query into another one, that you sort by date ascending
SELECT * FROM
(
    SELECT id, CONCAT(title, ' - ', content) AS "title - content", created_at
    FROM topics
    UNION ALL
    SELECT id, content, created_at
    FROM replies
    ORDER BY created_at DESC LIMIT 5
) subQuery
ORDER BY subQuery.created_at ASC;

输出

| id  | title - content   | created_at          |
| --- | ----------------- | ------------------- |
| 3   | content3          | 2019-02-05 19:14:43 |
| 4   | content4          | 2019-02-05 19:14:44 |
| 4   | title4 - content4 | 2019-02-05 19:14:44 |
| 5   | content5          | 2019-02-05 19:14:45 |
| 5   | title5 - content5 | 2019-02-05 19:14:45 |

View on DB Fiddle

在2个表格中获取最后5个帖子

2 个答案: