AddWithValue MySqlType.Decimal - mysql

Question

Unfortunately, I can't get any further. Working with vs2019 and vb. Created a cascading DropDownList (country, state, region). It works perfectly for country where I don't have to use AddWithValue (where). With state where I have to use AddWithValue (where), I don't get it baked. All IDs are defined as integers. The relevant code:
Protected Sub idLand_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
DropDownBundesland.Items.Clear()
DropDownBundesland.Items.Add(New ListItem("--Select Country--", ""))
DropDownRegion.Items.Clear()
DropDownRegion.Items.Add(New ListItem("--Select City--", ""))
DropDownBundesland.AppendDataBoundItems = True
Dim strConnString As [String] = ConfigurationManager _
.ConnectionStrings("conString").ConnectionString
Dim strQuery As [String] = "Select Bundesland, idBundesland, idLand from Campingplatz ORDER BY Bundesland ASC where idLand = #LandID Group BY Bundesland"
Dim con As New MySqlConnection(strConnString)
Dim cmd As New MySqlCommand()
cmd.Parameters.AddWithValue("#LandID", MySqlDbType.Decimal).Value = DropDownLand.SelectedItem.Value
cmd.CommandType = CommandType.Text
cmd.CommandText = strQuery
cmd.Connection = con
Try
con.Open()
DropDownBundesland.DataSource = cmd.ExecuteReader()
DropDownBundesland.DataTextField = "Bundesland"
DropDownBundesland.DataValueField = "idBundesland"
DropDownBundesland.DataBind()
If DropDownBundesland.Items.Count > 1 Then
DropDownBundesland.Enabled = True
Else
DropDownBundesland.Enabled = False
DropDownRegion.Enabled = False
End If
Catch ex As Exception
'Throw ex
Finally
con.Close()
con.Dispose()
End Try
End Sub
Please help!

Answer 1

this query is wrong:
Dim strQuery As [String] = "Select Bundesland, idBundesland, idLand from Campingplatz ORDER BY Bundesland ASC where idLand = #LandID Group BY Bundesland"
you must change the order of WHERE, ORDER BY and GROUP BY like:
Dim strQuery As [String] = "Select Bundesland, idBundesland, idLand from Campingplatz WHERE idLand = #LandID Group BY Bundesland ORDER BY Bundesland ASC"
Here you can see the Ssyntax: https://mariadb.com/kb/en/select/

Answer 2

I don't like to mix user interface code with database code. The database code should be independent of the kind of application where it is used.
Connections, Commands and DataReaders need to have their Dispose methods called so they can release unmanaged resources. You are provided with Using...End Using blocks to accomplish this and also close the connection.
You are mixing apples and oranges building the parameters collection. I showed both Add and AddWithValue in the code.
Please see my notes in the Catch block.
I would make the strConString a class level variable if the database is accessed by any other methods in the class.
Protected Sub idLand_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
DropDownBundesland.Items.Clear()
DropDownBundesland.Items.Add(New ListItem("--Select Country--", ""))
DropDownRegion.Items.Clear()
DropDownRegion.Items.Add(New ListItem("--Select City--", ""))
DropDownBundesland.AppendDataBoundItems = True
Dim dt As DataTable = Nothing
Try
dt = GetDropDownData(CDec(DropDownLand.SelectedItem.Value))
Catch ex As Exception
'Where would you be throwing it to? This is an event procedure so is not "called"
'Alert the user, maybe log the error
Exit Sub
End Try
DropDownBundesland.DataSource = dt
DropDownBundesland.DataTextField = "Bundesland"
DropDownBundesland.DataValueField = "idBundesland"
DropDownBundesland.DataBind()
If DropDownBundesland.Items.Count > 1 Then
DropDownBundesland.Enabled = True
Else
DropDownBundesland.Enabled = False
DropDownRegion.Enabled = False
End If
End Sub
Private strConnString As [String] = ConfigurationManager.ConnectionStrings("conString").ConnectionString
Private Function GetDropDownData(LandID As Decimal) As DataTable
Dim dt As New DataTable
Dim strQuery As [String] = "Select Bundesland, idBundesland, idLand
From Campingplatz
Where idLand = #LandID
Group BY Bundesland
ORDER BY Bundesland ASC "
Using con As New MySqlConnection(strConnString),
cmd As New MySqlCommand(strQuery, con)
cmd.Parameters.Add("#LandID", MySqlDbType.Decimal).Value = LandID
'or cmd.Parameters.AddWithValue("#LandID", LandID)
con.Open()
Using reader = cmd.ExecuteReader
dt.Load(reader)
End Using
End Using
Return dt
End Function