How Do I Use Credits.

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VB

Here is the story

I have a Mysql database setup:

Table Accounts:

Id: 1
Name: Donny
Credits: 600



and i have write a code with a button call's Buy

now i have on the button this little peace of code:

If credits.Text <= 0 Then
          MsgBox("Geen credits meer.")
      Else

          Dim conn As MySqlConnection
          conn = New MySqlConnection
          conn.ConnectionString = "server=xxx;port=3306; user id=xxx; password=xxx; database=xxx"
          Try
              conn.Open()
          Catch myerror As MySqlException
              MsgBox("error connecting to database")
          End Try
          Dim myadapter As New MySqlDataAdapter

          Dim sqlquery = "UPDATE account SET credits = credits -700 WHERE id='" & Trim(id.Text) & "'"

          Dim mycommand As New MySqlCommand()
          mycommand.Connection = conn
          mycommand.CommandText = sqlquery
          myadapter.SelectCommand = mycommand
          mycommand.ExecuteNonQuery()
          Try
              conn.Close()
          Catch myerror As MySqlException
              MessageBox.Show("Cannot connect to database: " & myerror.Message)
          Finally
              conn.Dispose()
          End Try

      End If

Now for a example i have on my account 600 credits.
and wanna buy a product it cost 700. now what i want is that the system says its not possible because you got only 600 credits .. now i have write a code that if its null then it show a message box but what is gonna happened is it show me the value: -100 and then if i click again then it shows the msg box.

How i must edit this peace of code its gonna happened what i want.