为什么将AVG(intger_column)转换为DECIMAL至少返回六位小数？

Question

考虑这个查询:

WITH Scores (score)
     AS
     (
      SELECT CAST(score AS INTEGER)
        FROM (
              VALUES (0), 
                     (10), 
                     (10)
             ) AS Scores (score)
     )
SELECT AVG(CAST(score AS DECIMAL(19, 8))) AS precision_eight, 
       AVG(CAST(score AS DECIMAL(19, 7))) AS precision_seven, 
       AVG(CAST(score AS DECIMAL(19, 6))) AS precision_six, 
       AVG(CAST(score AS DECIMAL(19, 5))) AS precision_five, 
       AVG(CAST(score AS DECIMAL(19, 4))) AS precision_four
  FROM Scores;

结果:

precision_eight | precision_seven | precision_six | precision_five | precision_four 
     6.66666666 |       6.6666666 |      6.666666 |       6.666666 |       6.666666 

为什么我总是得到至少六位小数？这是记录在案的行为？

(我正在运行SQL Server 2008)

Answer 1

小数的AVG始终返回"十进制(38,s)除以十进制(10,0)"数据类型(请参见此处)

您必须将AVG结果转换为所需的精度.