而不是 MySQL ORDER BY RAND

Question

我使用这个查询：

SELECT f.id FROM articles f
JOIN ( SELECT RAND() * (SELECT MAX(id) FROM articles) AS max_id ) AS m
WHERE f.id >= m.max_id
ORDER BY f.id ASC
LIMIT 5

问题在于它返回了意外的行数。它可以是 1 行或 3 行或 5 行。但我正好需要 5 行。

Answer 1

您的查询可能存在的问题是它仅随机选择一条记录，而其余记录是连续的。如果你想真正随机选择所有5个，你必须一个一个地选择它们：

(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random = f.id) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random = f.id) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random = f.id) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random = f.id) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random = f.id)

如果 ID 是连续的且没有间隔，则此方法有效。如果有差距，则必须将等式更改为小于，并限制为一条记录：

(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random <= f.id LIMIT 1) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random <= f.id LIMIT 1) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random <= f.id LIMIT 1) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random <= f.id LIMIT 1) UNION ALL
(SELECT f.id FROM articles f INNER JOIN (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM articles) + 1) AS random) m ON m.random <= f.id LIMIT 1)

但是，就像 dnswlt 在他的answer 中提到的那样。更简单的经典解决方案适用于小表：

SELECT f.id
FROM articles f
ORDER BY RAND()
LIMIT 5

Answer 2

您的问题看起来与MySQL select 10 random rows from 600K rows fast 重复。

根据您的表中有多少行以及您愿意接受的响应时间，您可以先尝试经典变体：

SELECT f.id FROM articles f
ORDER BY RAND()
LIMIT 5

如果少于一百万，这可能就足够了。

否则你可以应用链接文章中描述的技巧，以及UNION ALL 5 个这样的查询，大致如下：

(SELECT r1.id
  FROM articles AS r1 JOIN
       (SELECT CEIL(RAND() *
                     (SELECT MAX(id)
                        FROM articles)) AS id)
        AS r2
 WHERE r1.id >= r2.id
 ORDER BY r1.id ASC
 LIMIT 1)
 UNION ALL
 (SELECT r1.id
  FROM articles AS r1 JOIN
       (SELECT CEIL(RAND() *
                     (SELECT MAX(id)
                        FROM articles)) AS id)
        AS r2
 WHERE r1.id >= r2.id
 ORDER BY r1.id ASC
 LIMIT 1)
 UNION ALL
 ... (three more queries to retrieve 5 rows)

如果您的行数很少，则此解决方案中可能会出现重复。但是，如果您的行数很少，则无论如何都可以使用上面更简单的解决方案:)

我为第二个变体创建了SQL Fiddle。 HTH！