要小心，因为TableSample实际上并不返回随机的行样本。它引导您的查询查看组成行的8KB页面的随机样本。然后，对这些页面中包含的数据执行查询。由于数据在这些页面上的分组方式(插入顺序等)，这可能导致数据实际上不是随机样本。

参见:http://www.mssqltips.com/tip.asp?tip=1308

该表的MSDN页面包含了如何生成实际随机数据样本的示例。

http://msdn.microsoft.com/en-us/library/ms189108.aspx

对于SQL Server 2005和2008，如果我们想要一个随机的个别行样本(来自Books Online):

SELECT * FROM Sales.SalesOrderDetail
WHERE 0.01 >= CAST(CHECKSUM(NEWID(), SalesOrderID) & 0x7fffffff AS float)
/ CAST (0x7fffffff AS int)

对于SQL Server和需要“单个随机行”..

如果不需要真采样，生成一个随机值[0,max_rows)并使用ORDER BY..OFFSET..从SQL Server 2012+获取。

如果COUNT和ORDER BY在适当的索引上，这是非常快的——这样数据就已经沿着查询行“排序”了。如果涵盖了这些操作，那么它就是一个快速请求，并且不会受到使用ORDER BY NEWID()或类似方法的可怕可伸缩性的影响。显然，这种方法在非索引的HEAP表上不能很好地伸缩。

declare @rows int
select @rows = count(1) from t

-- Other issues if row counts in the bigint range..
-- This is also not 'true random', although such is likely not required.
declare @skip int = convert(int, @rows * rand())

select t.*
from t
order by t.id -- Make sure this is clustered PK or IX/UCL axis!
offset (@skip) rows
fetch first 1 row only

确保使用了适当的事务隔离级别和/或考虑0结果。

对于SQL Server，需要一个“一般行样本”的方法..

注意:这是一个在SQL Server上找到的关于获取行样本的特定问题的答案的改编。它是根据上下文量身定制的。

虽然这里应该谨慎使用一般抽样方法，但对于其他答案(以及关于非伸缩和/或有问题的实现的重复建议)，它仍然是潜在的有用信息。如果目标是找到“单个随机行”，那么这种抽样方法的效率低于所示的第一个代码，并且容易出错。

这是一个更新和改进的对行百分比进行抽样的形式。它基于与其他一些使用CHECKSUM / BINARY_CHECKSUM和modulus的答案相同的概念。

It is relatively fast over huge data sets and can be efficiently used in/with derived queries. Millions of pre-filtered rows can be sampled in seconds with no tempdb usage and, if aligned with the rest of the query, the overhead is often minimal. Does not suffer from CHECKSUM(*) / BINARY_CHECKSUM(*) issues with runs of data. When using the CHECKSUM(*) approach, the rows can be selected in "chunks" and not "random" at all! This is because CHECKSUM prefers speed over distribution. Results in a stable/repeatable row selection and can be trivially changed to produce different rows on subsequent query executions. Approaches that use NEWID() can never be stable/repeatable. Does not use ORDER BY NEWID() of the entire input set, as ordering can become a significant bottleneck with large input sets. Avoiding unnecessary sorting also reduces memory and tempdb usage. Does not use TABLESAMPLE and thus works with a WHERE pre-filter.

这是要点。有关更多细节和注意事项，请参阅这个答案。

Naï亿一下:

declare @sample_percent decimal(7, 4)
-- Looking at this value should be an indicator of why a
-- general sampling approach can be error-prone to select 1 row.
select @sample_percent = 100.0 / count(1) from t

-- BAD!
-- When choosing appropriate sample percent of "approximately 1 row"
-- it is very reasonable to expect 0 rows, which definitely fails the ask!
-- If choosing a larger sample size the distribution is heavily skewed forward,
-- and is very much NOT 'true random'.
select top 1
    t.*
from t
where 1=1
    and ( -- sample
        @sample_percent = 100
        or abs(
            convert(bigint, hashbytes('SHA1', convert(varbinary(32), t.rowguid)))
        ) % (1000 * 100) < (1000 * @sample_percent)
    )

这可以在很大程度上通过混合抽样和ORDER by从小得多的样本集中选择的混合查询来补救。这将排序操作限制为样本大小，而不是原始表的大小。

-- Sample "approximately 1000 rows" from the table,
-- dealing with some edge-cases.
declare @rows int
select @rows = count(1) from t

declare @sample_size int = 1000
declare @sample_percent decimal(7, 4) = case
    when @rows <= 1000 then 100                              -- not enough rows
    when (100.0 * @sample_size / @rows) < 0.0001 then 0.0001 -- min sample percent
    else 100.0 * @sample_size / @rows                        -- everything else
    end

-- There is a statistical "guarantee" of having sampled a limited-yet-non-zero number of rows.
-- The limited rows are then sorted randomly before the first is selected.
select top 1
    t.*
from t
where 1=1
    and ( -- sample
        @sample_percent = 100
        or abs(
            convert(bigint, hashbytes('SHA1', convert(varbinary(32), t.rowguid)))
        ) % (1000 * 100) < (1000 * @sample_percent)
    )
-- ONLY the sampled rows are ordered, which improves scalability.
order by newid()

我不知道这有多有效，但我以前用过:

SELECT TOP 1 * FROM MyTable ORDER BY newid()

因为guid是非常随机的，所以顺序意味着您得到的是随机行。

我还没看出来答案有什么不同。我有一个额外的约束条件，给定一个初始种子，每次都要选择相同的行集。

对于MS SQL:

最小的例子:

select top 10 percent *
from table_name
order by rand(checksum(*))

规范化执行时间:1.00

NewId()例子:

select top 10 percent *
from table_name
order by newid()

规范化执行时间:1.02

NewId()比rand(checksum(*))慢不了多少，所以您可能不希望对大型记录集使用它。

初始种子选择:

declare @seed int
set @seed = Year(getdate()) * month(getdate()) /* any other initial seed here */

select top 10 percent *
from table_name
order by rand(checksum(*) % seed) /* any other math function here */

如果给定一个种子，你需要选择相同的集合，这似乎是可行的。

一个简单而有效的方法从http://akinas.com/pages/en/blog/mysql_random_row/

SET @i = (SELECT FLOOR(RAND() * COUNT(*)) FROM table); PREPARE get_stmt FROM 'SELECT * FROM table LIMIT ?, 1'; EXECUTE get_stmt USING @i;