mysql - MySQL 限制 OR

Question

我想知道 MySQL 中是否有一种方法可以为每个 OR 限制 1。例如，如果我有一个 MySQL 语句SELECT * FROM posts WHERE username = 'Johnny' OR username = 'Matthew' OR username = 'Andy' OR username = 'Sandra' ORDER BY date DESC，但我只想从每个用户名返回一个结果（或者，在这种情况下，每个人的最新帖子）。我已经尝试过SELECT * FROM posts WHERE (username = 'Johnny' LIMIT 1) OR (username = 'Matthew' LIMIT 1) OR (username = 'Andy' LIMIT 1) OR (username = 'Sandra' LIMIT 1) ORDER BY date DESC了，但这就是我能想到的（但它没有用）。在此先感谢，马修

Answer 1

您可以使用UNION组合单独的查询：

(SELECT * FROM posts WHERE username = 'Johnny'  LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Matthew' LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Andy'    LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Sandra'  LIMIT 1)

或者，您可以按用户名对结果进行分组：

SELECT   *
FROM     posts
WHERE    username IN ('Johnny','Matthew','Andy','Sandra')
GROUP BY username

上面给出的这两个查询都将为每个用户返回不确定的行（这可能是您需要的，也可能不是您需要的）。要获取特定行，请指定 MySQL 如何确定您想要的行。

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更新

要获取每个用户的最新帖子并将结果集从最新到最旧排序：

(SELECT * FROM posts WHERE username = 'Johnny'  ORDER BY date DESC LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Matthew' ORDER BY date DESC LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Andy'    ORDER BY date DESC LIMIT 1)
UNION ALL
(SELECT * FROM posts WHERE username = 'Sandra'  ORDER BY date DESC LIMIT 1)
ORDER BY date DESC

和：

SELECT   *
FROM     posts NATURAL JOIN (
           SELECT   username, MAX(date) AS date
           FROM     posts
           WHERE    username IN ('Johnny','Matthew','Andy','Sandra')
           GROUP BY username
         ) AS t
GROUP BY username -- only required if multiple posts might have the same date
ORDER BY date DESC

Answer 2

如何使用联合？

感谢@eggyal更正编辑：

(SELECT * FROM posts WHERE username = 'Johnny' LIMIT 1)
UNION
(SELECT * FROM posts WHERE username = 'Matthew' LIMIT 1)
UNION
(SELECT * FROM posts WHERE username = 'Andy' LIMIT 1)