從每個組中選擇隨機值SQL

Question

我有一個項目，通過它我可以創建一個由數據庫驅動的游戲。

數據庫輸入的數據如下：

（身份證，姓名）|| （1，PhotoID），（1，PhotoID），（1，PhotoID），（2，PhotoID），（2，PhotoID）等。 有數以千計的條目。

這是我當前的SQL語句：

$sql = "SELECT TOP 8 * FROM Image WHERE Hidden = '0' ORDER BY NEWID()";

但是這也可以產生具有匹配ID的結果，其中我需要使每個結果具有唯一ID（即我需要來自每個組的一個結果）。

如何更改查詢以從每個組中獲取一個結果？

謝謝！

Answer 1

由於ORDER BY NEWID()無論如何都會導致表掃描，因此您可以使用row_number（）來隔離組中的第一個：

; with randomizer as (
  select id,
         name,
         row_number() over (partition by id
                            order by newid()) rn
    from Image
   where hidden = 0
)
select top 8
       id,
       name
  from randomizer
 where rn = 1
-- Added by mellamokb's suggestion to allow groups to be randomized
order by newid()

由於mellamokb， Sql Fiddle游樂場 。

Answer 2

看起來這可能有用，但我無法保證性能：

SELECT TOP 8 ID,
  (select top 1 name from image i2
   where i2.id = i1.id order by newid())
FROM Image i1
WHERE hidden = '0'
group by ID
ORDER BY NEWID();

演示： http ：//www.sqlfiddle.com/#！3 / 657ad / 6

Answer 3

如果您在ID列上有索引並希望利用索引並避免全表掃描，請首先對鍵值進行隨機化：

WITH IDs AS
(
  SELECT DISTINCT ID
  FROM Image
  WHERE Hidden = '0'
),
SequencedIDs AS
(
  SELECT ID, ROW_NUMBER() OVER (ORDER BY NEWID()) AS Seq
  FROM IDs
),
ImageGroups AS
(
  SELECT i.*, ROW_NUMBER() OVER (PARTITION BY i.ID ORDER BY NEWID()) Seq
  FROM SequencedIDs s
  INNER JOIN Image i
    ON i.ID = s.ID
  WHERE s.Seq < 8
  AND i.Hidden = '0'
)
SELECT *
FROM ImageGroups
WHERE Seq = 1

這應該大大降低了表掃描方法的成本，雖然我沒有足夠大的模式可以測試 - 所以嘗試在SSMS中運行一些統計信息並確保ID實際上被索引以使其有效。

Answer 4

select * from (select * from photos order by rand()) as _SUB group by _SUB.id;

Answer 5

 select ID, Name from (select ID, Name, row_number() over 
 (partition by ID, Name order by ID) as ranker from Image where Hidden = 0 ) Z where ranker = 1
 order by newID()