Question

如何根据所有候选行的应用权重在T-SQL中随机选择一个表行？

例如，我在一个表格中有一组行，加权为50,25和25（最多加100但不需要），我想随机选择其中一行，统计结果等价各自的重量。

Answer 1

Dane的回答包括以引入平方定律的方式进行自我连接。联接后(n*n/2)行，表中有n行。

更理想的是能够只解析一次表。

DECLARE @id int, @weight_sum int, @weight_point int
DECLARE @table TABLE (id int, weight int)

INSERT INTO @table(id, weight) VALUES(1, 50)
INSERT INTO @table(id, weight) VALUES(2, 25)
INSERT INTO @table(id, weight) VALUES(3, 25)

SELECT @weight_sum = SUM(weight)
FROM @table

SELECT @weight_point = FLOOR(((@weight_sum - 1) * RAND() + 1), 0)

SELECT
    @id = CASE WHEN @weight_point < 0 THEN @id ELSE [table].id END,
    @weight_point = @weight_point - [table].weight
FROM
    @table [table]
ORDER BY
    [table].Weight DESC

这将通过表格，将@id设置为每个记录的id值，同时递减@weight点。最终，@weight_point将变为负数。这意味着所有先前权重的SUM大于随机选择的目标值。这是我们想要的记录，因此从那时起我们将@id设置为自身（忽略表中的任何ID）。

这只会在表中运行一次，但即使所选值是第一条记录，也必须遍历整个表。因为平均位置是表格的一半（如果按递增权重排序则更少）写一个循环可能会更快......（特别是如果权重是共同的组）：

DECLARE @id int, @weight_sum int, @weight_point int, @next_weight int, @row_count int
DECLARE @table TABLE (id int, weight int)

INSERT INTO @table(id, weight) VALUES(1, 50)
INSERT INTO @table(id, weight) VALUES(2, 25)
INSERT INTO @table(id, weight) VALUES(3, 25)

SELECT @weight_sum = SUM(weight)
FROM @table

SELECT @weight_point = ROUND(((@weight_sum - 1) * RAND() + 1), 0)

SELECT @next_weight = MAX(weight) FROM @table
SELECT @row_count   = COUNT(*)    FROM @table
SET @weight_point = @weight_point - (@next_weight * @row_count)

WHILE (@weight_point > 0)
BEGIN
    SELECT @next_weight = MAX(weight) FROM @table WHERE weight < @next_weight
    SELECT @row_count   = COUNT(*)    FROM @table WHERE weight = @next_weight
    SET @weight_point = @weight_point - (@next_weight * @row_count)
END

-- # Once the @weight_point is less than 0, we know that the randomly chosen record
-- # is in the group of records WHERE [table].weight = @next_weight

SELECT @row_count = FLOOR(((@row_count - 1) * RAND() + 1), 0)

SELECT
    @id = CASE WHEN @row_count < 0 THEN @id ELSE [table].id END,
    @row_count = @row_count - 1
FROM
    @table [table]
WHERE
    [table].weight = @next_weight
ORDER BY
    [table].Weight DESC

Answer 2

您只需要对所有候选行的权重求和，然后在该总和中选择一个随机点，然后选择与该选定点协调的记录（每个记录逐渐带有累加权重和）。

DECLARE @id int, @weight_sum int, @weight_point int
DECLARE @table TABLE (id int, weight int)

INSERT INTO @table(id, weight) VALUES(1, 50)
INSERT INTO @table(id, weight) VALUES(2, 25)
INSERT INTO @table(id, weight) VALUES(3, 25)

SELECT @weight_sum = SUM(weight)
FROM @table

SELECT @weight_point = ROUND(((@weight_sum - 1) * RAND() + 1), 0)

SELECT TOP 1 @id = t1.id
FROM @table t1, @table t2
WHERE t1.id >= t2.id
GROUP BY t1.id
HAVING SUM(t2.weight) >= @weight_point
ORDER BY t1.id

SELECT @id

Answer 3

如果你有很多记录，那么“逐步携带一个累积的[sic]权重和”部分是很昂贵的。如果你已经有很多分数/重量（即：范围很宽，大多数记录权重都是独一无二的.1-5颗星可能不会削减它），你可以做这样的事情来选择一个重量值。我在这里使用VB.Net进行演示，但这也可以在纯Sql中轻松完成：

Function PickScore()
    'Assume we have a database wrapper class instance called SQL and seeded a PRNG already
    'Get count of scores in database
    Dim ScoreCount As Double = SQL.ExecuteScalar("SELECT COUNT(score) FROM [MyTable]")
    ' You could also approximate this with just the number of records in the table, which might be faster.

    'Random number between 0 and 1 with ScoreCount possible values
    Dim rand As Double = Random.GetNext(ScoreCount) / ScoreCount

    'Use the equation y = 1 - x^3 to skew results in favor of higher scores
    ' For x between 0 and 1, y is also between 0 and 1 with a strong bias towards 1
    rand = 1 - (rand * rand * rand)

    'Now we need to map the (0,1] vector to [1,Maxscore].
    'Just find MaxScore and mutliply by rand
    Dim MaxScore As UInteger = SQL.ExecuteScalar("SELECT MAX(Score) FROM Songs")
    Return MaxScore * rand
End Function

运行此选项，并选择分数小于返回权重的记录。如果多个记录共享该分数，则随机选择。这里的优点是你不必保留任何总和，你可以调整用于满足你的口味的概率方程。但同样，它在分数分布较大时效果最佳。

Answer 4

使用随机数生成器执行此操作的方法是集成概率密度函数。使用一组离散值，您可以计算前缀总和（所有值的总和，直到此值）并存储它。使用此选项，您可以选择最大前缀总和（聚合到日期）值大于随机数。

在数据库中，必须更新插入后的后续值。如果更新的相对频率和数据集的大小没有使得这样做的成本过高，则意味着可以从单个s-argable（可以通过索引查找解析的谓词）查询中获得适当的值

Answer 5

如果需要获取一组样本（例如，您要从5M行的集合中抽取50行），其中每行都有一个名为Content-Security-Policy: default-src 'none'; script-src 'self' www.google-analytics.com; connect-src 'self'; img-src 'self' www.google-analytics.com; style-src 'self' fonts.googleapis.com; font-src 'self' fonts.gstatic.com;的列，该列为Weight，如果值越大表示体重越重，则可以使用此功能：

int

这里的关键是使用POWER（）函数，如图here

选择随机函数的参考是here和here

或者，您可以使用：

SELECT * 
FROM 
(
    SELECT TOP 50 RowData, Weight 
    FROM MyTable 
    ORDER BY POWER(RAND(CAST(NEWID() AS VARBINARY)), (1.0/Weight)) DESC
) X 
ORDER BY Weight DESC

由于this问题，您将校验和转换为1.0 * ABS(CAST(CHECKSUM(NEWID()) AS bigint)) / CAST(0x7FFFFFFF AS INT)而不是BIGINT：

因为校验和返回一个int，并且int的范围是-2 ^ 31 （-2,147,483,648）到2 ^ 31-1（2,147,483,647），abs（）函数可以如果结果恰好是正确，则返回一个溢出错误 -2,147,483,648！机会显然很低，大约为40亿分之一，但是我们每天在〜1.8b的行表上运行它，因此大约一周一次！解决方法是将校验和转换为 bigint在腹肌之前。

T-SQL中的随机加权选择

5 个答案: