Question

我有一个项目，我正在创建一个由数据库驱动的游戏。

数据库输入的数据如下：

（身份证，姓名）|| （1，PhotoID），（1，PhotoID），（1，PhotoID），（2，PhotoID），（2，PhotoID）等。有数以千计的参赛作品。

这是我当前的SQL语句：

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

但是这也会产生匹配ID的结果，我需要让每个结果都有一个唯一的ID（我需要每个组得到一个结果）。

如何更改查询以从每个组中获取一个结果？

谢谢！

Answer 1

由于ORDER BY NEWID()无论如何都会导致表扫描，你可以使用row_number（）来隔离第一组：

; 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()

Sql Fiddle playground感谢mellamokb。

Answer 2

看起来这可行，但我无法保证性能：

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列上有索引并希望利用索引并避免全表扫描，请先对关键值进行随机化：

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

这应该大大降低了表扫描方法的成本，虽然我没有足够大的模式可以测试 - 所以尝试在SSMS中运行一些统计信息并确保ID实际上已为其编制索引这是有效的。

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()

从每个组中选择随机值SQL

5 个答案: