Mysql 而不是按兰德订购

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

---

问题是它返回了意外的行数。它可以是1行、3行或5行。但我只需要5行。

您的查询可能存在的一个问题是，它只随机选择一条记录，其余记录是连续的。如果要真正随机选择全部5个，则必须逐个选择：

(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是连续的且没有间隙，则此操作有效。如果存在间隙，则必须将相等值更改为小于，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)

---

你的问题看起来像是重复的

根据您的表中有多少行以及您愿意接受的响应时间，您可以首先尝试经典的变体：

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

如果少于一百万，这就足够了

否则，您可以应用链接文章中描述的技巧，并

UNION ALL

5个这样的查询，大致如下所示：

(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存在重复。但如果行数不多，可以使用上面更简单的解决方案：）

---

我为第二个变体创建了一个。嗯

你想达到什么目标？选择5个随机行？选择5个连续的行（按id排序，从随机位置开始）？我想选择5个随机行您的五个连接将返回0或1行，最有可能是0，除非对

RAND

的所有五个调用都产生相同的值：）@dnswlt当然。它需要这些子查询之间的联合。我不知道查询结果如何。我想我累了，复制/粘贴了内部子查询而不是外部子查询，忘记了添加联合。谢谢你的来信。

(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)