從“ DBNull”類型到“ Double”類型的轉換無效

Question

在我的代碼中，我理解查詢獲取空值，並引發此錯誤。 但是由於我的查詢有點復雜，所以我不明白如何檢查空值並避免此錯誤。 請幫助我更正此查詢。

SELECT (SUM(charges) + SUM(behaviour) + SUM(admission) + SUM(properInformation) + SUM(hygine) + SUM(treatment))/(count(doctorID) * 6) AverageRating, COUNT(ID) RatingCount from ratings where doctorID = '" + doctorID + "'

Answer 1

如果您希望查詢不返回NULL ，則可以將表達式用IFNULL包圍，以將可能的NULL轉換為0，例如：

SELECT IFNULL((SUM(charges) + SUM(behaviour) + SUM(admission) + 
               SUM(properInformation) + SUM(hygine) + SUM(treatment))
              /(count(doctorID) * 6), 0) AverageRating, 
       COUNT(ID) RatingCount 
FROM ratings 
WHERE doctorID = '" + doctorID + "'

Answer 2

如果您確實知道查詢正確返回了空值，則可以使用try-catch塊，如下所示：

Try
Dim dt As DataTable = Me.GetData("SELECT (SUM(charges) + SUM(behaviour) + SUM(admission) + SUM(properInformation)
Catch ex As Exception
MsgBox("Error while fetching data" & vbCrLf & ex.Message)
End Try