MySQL随机排序分区

表1在我的数据库中有3列：id、category、timestamp。我需要查询每个类别中最新的3行：

WITH ranked_rows AS
(SELECT t.*, ROW_NUMBER() OVER (PARTITION BY category ORDER BY t.timestamp DESC) AS rn
FROM table1 AS t)
SELECT ranked_rows.* FROM ranked_rows WHERE rn<=3

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现在我需要从结果中随机选择10个分区。请注意，每个分区有3行。如何做到这一点？

有多种方法。一是:

WITH ranked_rows AS (
      SELECT t.*,
             ROW_NUMBER() OVER (PARTITION BY category ORDER BY t.timestamp DESC) AS seqnum,
             DENSE_RANK() OVER (ORDER BY MD5(category)) as catnum
      FROM table1 t
     )
SELECT ranked_rows.*
FROM ranked_rows
WHERE seqnum <= 3 AND catnum <= 10;

---

md5只会使结果看起来随机。

如果您希望每个类别都是真正随机的，这里有一种方法：

with categorycte as (
  select category , rand() randomcatid
  from table1
  group by category
),ranked_rows AS
(
  SELECT t.*
  , ROW_NUMBER() OVER (PARTITION BY category ORDER BY t.timestamp DESC) AS rn
  , dense_rank() over (order by randomcatid) catnum
FROM table1 AS t
join categorycte c on t.category = c.category
)
SELECT ranked_rows.* FROM ranked_rows 
WHERE rn<=3 and catnum <= 10;

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非常感谢！但是每次我运行这个查询时，每个分区的catnum值都不会改变，因此结果是常量。@Soheil。这可能是一个特性，但您现在可以对md5:md5concatcatcatcategory添加盐。