我有一个循环，它将遍历所有唯一ID以查找现有记录。如果在循环中有一个带有该数字的ID，那么我希望它做某事，否则我希望它跳到下一个数字。我有一个柜台，所以我可以做那个部分。我遇到的问题是，当它没有遇到ID时，它会在尝试将任何内容放入有意义的变量时出错。如果数字不存在，我如何才能进入下一个数字。箭头出现错误。我的代码如下：
Sub Cmd_CalculateClick(sender As Object, e As EventArgs)
weekcount = 0
'Calculate Bills for whole term
Do Until counter = 9999
valid_user = 0
SQLcommand = SQLconnect.CreateCommand
'Create SQL statement
SQLcommand.CommandText = "SELECT Pupil_ID FROM Pupil WHERE Pupil_ID = '" & counter & "' AND Waiting_List = 'F' "
'Extract data
SQLreader = SQLcommand.ExecuteReader()
----> valid_user = SQLreader("Pupil_ID")
'Clear SQL command buffer
SQLcommand.Dispose()
If valid_user = 0 Then
counter = counter + 1
Else If valid_user > 0 Then
'Fetch Pupil attendance Days
SQLcommand = SQLconnect.CreateCommand
'Create SQL statement
SQLcommand.CommandText = "SELECT Attendance_Monday, Attendance_Tuesday, Attendance_Wednesday, Attendance_Thursday, Attendance_Friday FROM Pupil WHERE Pupil_ID = '" & valid_user & "'"
'Extract data
SQLreader = SQLcommand.ExecuteReader()
If SQLreader("Attendance_Monday") = "T" Then
Monday = True
weekcount = weekcount + 1
Else
Monday = False
End If
If SQLreader("Attendance_Tuesday") = "T" Then
Tuesday = True
weekcount = weekcount + 1
Else
Tuesday = False
End If
If SQLreader("Attendance_Wednesday") = "T" Then
Wednesday = True
weekcount = weekcount + 1
Else
Wednesday = False
End If
If SQLreader("Attendance_Thursday") = "T" Then
Thursday = True
weekcount = weekcount + 1
Else
Thursday = False
End If
If SQLreader("Attendance_Friday") = "T" Then
Friday = True
weekcount = weekcount + 1
Else
Friday = False
End If
'Clear SQL command buffer
SQLcommand.Dispose()
'Fetch Total Sessions for that half term
SQLcommand = SQLconnect.CreateCommand
'Create SQL statement
SQLcommand.CommandText = "SELECT Session_Monday, Session_Tuesday, Session_Wednesday, Session_Thursday, Session_Friday FROM Session"
'Extract data
SQLreader = SQLcommand.ExecuteReader()
sesmon = SQLreader("Session_Monday")
sestue = SQLreader("Session_Tuesday")
sesmon = SQLreader("Session_Wednesday")
sesmon = SQLreader("Session_Thursday")
sesmon = SQLreader("Session_Friday")
'Clear SQL command buffer
SQLcommand.Dispose()
'Fetch Cost for each session
SQLcommand = SQLconnect.CreateCommand
'Create SQL statement
SQLcommand.CommandText = "SELECT Price_Monday, Price_Tuesday, Price_Wednesday, Price_Thursday, Price_Friday FROM Price"
'Extract data
SQLreader = SQLcommand.ExecuteReader()
Cost_Monday = SQLreader("Price_Monday")
Cost_Tuesday = SQLreader("Price_Tuesday")
Cost_Wednesday = SQLreader("Price_Wednesday")
Cost_Thursday = SQLreader("Price_Thursday")
Cost_Friday = SQLreader("Price_Friday")
'Clear SQL command buffer
SQLcommand.Dispose()
'Calculate Cost per day
If Monday = True Then
Total_Monday = sesmon * Cost_Monday
Else
Total_Monday = 0
End If
If Tuesday = True Then
Total_Tuesday = sestue * Cost_Tuesday
Else
Total_Tuesday = 0
End If
If Wednesday = True Then
Total_Wednesday = seswed * Cost_Wednesday
Else
Total_Wednesday = 0
End If
If Thursday = True Then
Total_Thursday = sesthur * Cost_Thursday
Else
Total_Thursday = 0
End If
If Friday = True Then
Total_Friday = sesfri * Cost_Friday
Else
Total_Friday = 0
End If
'Calculate week cost
weekcost = weekcount * Cost_Monday
'calculate term cost
termtotal = Total_Monday + Total_Tuesday + Total_Wednesday + Total_Thursday + Total_Friday
counter = counter + 1
End If
Loop
End Sub
由于
答案 0 :(得分：1)
您可以将所有这些内容压缩为单个sql语句，并极大地减少您的代码：
Sub Cmd_CalculateClick(sender As Object, e As EventArgs)
Dim sql As String = _
"SELECT Pupil_ID, " & _
"CASE WHEN Attendance_Monday = 'T' THEN p.Price_Monday ELSE 0 END " & _
"+ CASE WHEN Attendance_Tuesday = 'T' THEN p.Price_Tuesday ELSE 0 END " & _
"+ CASE WHEN Attendance_Wednesday = 'T' THEN p.Price_Wednesday ELSE 0 END " & _
"+ CASE WHEN Attendance_Thursday = 'T' THEN p.Price_Thursday ELSE 0 END " & _
"+ CASE WHEN Attendance_Friday = 'T' THEN p.Price_Friday ELSE 0 END " & _
"As weekTotal, " & _
"CASE WHEN Attendance_Monday = 'T' THEN p.Price_Monday * Session_Monday ELSE 0 END " & _
"+ CASE WHEN Attendance_Tuesday = 'T' THEN p.Price_Tuesday * Session_Tuesday ELSE 0 END " & _
"+ CASE WHEN Attendance_Wednesday = 'T' THEN p.Price_Wednesday * Session_Wednesday ELSE 0 END " & _
"+ CASE WHEN Attendance_Thursday = 'T' THEN p.Price_Thursday * Session_Thursday ELSE 0 END " & _
"+ CASE WHEN Attendance_Friday = 'T' THEN p.Price_Friday * Session_Friday ELSE 0 END " & _
"As termTotal " & _
"FROM Pupil p " & _
"INNER JOIN Session s ON 1=1 " & _
"INNER JOIN Price p ON 1=1 " & _
"WHERE p.Waiting_List = 'F'"
Using cn As New SqlConnection("connection string here"), _
cmd As New SqlCommand(sql, cn)
cn.Open()
Using rdr As SqlDataReader = cmd.ExecuteReader()
While rdr.Read()
Dim pupilID As Integer = Convert.ToInt32(rdr("Pupil_ID"))
Dim weekTotal As Decimal = Convert.ToDecimal(rdr("weekTotal"))
Dim termTotal As Decimal = Convert.ToDecimal(rdr("termTotal"))
End While
End Using
End Using
End Sub
当然我不知道你想用这些数字做什么，但是如果你只是想把它写回某个地方的数据库，你甚至可以在查询中包含它，而且永远不需要将结果全部读入VB端。这将极大地提高您的表现。
答案 1 :(得分：0)
if(SQLreader.HasRows)
{
//....
}
在尝试从中获取Pupil_ID之前尝试此操作。
答案 2 :(得分：0)
您可以更改代码以使用ExecuteScalar并使用参数化查询
SQLcommand.CommandText = "SELECT Pupil_ID FROM Pupil " & _
"WHERE Pupil_ID = @cnt AND Waiting_List = 'F' "
SQLcommand.Parameters.AddWithValue("@cnt", counter.ToString)
Dim result = SQLcommand.ExecuteScalar()
if result Is Nothing then
Continue Do
else
valid_user = Convert.ToInt32(result)
.... continue with the rest of your code
这将有助于提高性能，因为ExecuteScalar可能比创建SqlDataReader的实例更快。在Do ...循环外创建命令并更新循环内的参数值
，可以实现另一点性能提升Sub Cmd_CalculateClick(sender As Object, e As EventArgs)
weekcount = 0
SQLcommand = SQLconnect.CreateCommand
SQLcommand.CommandText = "SELECT Pupil_ID FROM Pupil " & _
"WHERE Pupil_ID = @cnt AND Waiting_List = 'F' "
SQLcommand.Parameters.AddWithValue("@cnt", "") 'dummy value here'
Do Until counter = 9999
valid_user = 0
SQLcommand.Parameters("@cnt").Value = counter.ToString
Dim result = SQLcommand.ExecuteScalar()
if result Is Nothing then
Continue Do
else
valid_user = Convert.ToInt32(result)
.... continue with the rest of your code
.... but not destroy the command until the loop exit