爲什麼將AVG（intger_column）作爲DECIMAL返回最少六位小數？

Question

考慮這個查詢：

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; 

結果：

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

爲什麼我總是得到至少6位小數？這是記錄的行爲？

（我運行SQL Server 2008）

Answer 1

小數的AVG總是返回一個 「以十進制小數分（38，S）（10,0）」 的數據類型（see here）

你有將AVG結果轉換爲期望的精度。

Answer 2

我正要發佈類似於Jappie's answer的東西以及這個例子來說明幕後發生了什麼。

declare @a int 
set @a = 0 
declare @b int 
set @b = 10 
declare @c int 
set @c = 10 

declare @n decimal(10,0) 
set @n = 3 

select cast((@a+@b+@c) as decimal(38,4))/@n