Question

我在MySQL中有一个表：

Col1 | Col2 
 a       A
 a       B 
 c       C
 a       B

我想创建一个这样的表：

col1 | col2 |    freq
 a        A      0.33
 a        B      0.67

col1是Col1中的指定项目。 col2是与指定项目（即a）发生的不同项目。 freq列是col2中项目的出现频率。

有人能给我一些如何创建这样一个查询的提示吗？非常感谢。

Answer 1

试试这个：

Select  A.Col1, A.Col2, A.Count1 * 1.0 / B.Count2 As Freq
From    (
        Select Col1, Col2, Count(*) As Count1
        From   YourTableName
        Group By Col1, Col2
        ) As A
        Inner Join (
            Select Col1, Count(*) As Count2
            From   YourTableName
            Group By Col1
            ) As B
            On A.Col1 = B.Col1

Answer 2

您也可以使用在SQL Server中编码的

  DECLARE @Count INT;
  SELECT @Count = COUNT(1) FROM YourTableName   WHERE Col1 = 'a'

  SELECT Col1, Col2, CAST(COUNT(1)* 1.00 /@Count AS DECIMAL(4,2) ) AS Frequency
  FROM YourTableName  
  WHERE Col1 = 'a'
  GROUP BY Col1, Col2

这样你就有了更好的表现

Answer 3

带窗口函数

SELECT Col1, Col2, Count1*1.0 / Count2 AS freq
FROM (
    SELECT 
        Col1, 
        Col2,
        COUNT() OVER(PARTITION BY Col1, Col2) AS Count1,
        COUNT() OVER(PARTITION BY Col1) AS Count2
    FROM YourTableName
    )
GROUP BY Col1, Col2

使用sql计算频率

3 个答案: