Detecting if data exists for OleDB COUNT query - sql

Question

I'm trying to pull data from ACCESS database.
As it is, the code works, and gives no errors...
However, I can't seem to be able to display a messagebox if the record doesn't exist. It simply returns an empty string.
Using dbCon = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;" & "Data Source = '" & Application.StartupPath & "\Res\T500G.accdb'")
dbCon.Open()
Dim Query1 As String = "SELECT SUM(Total) FROM [T500] WHERE Pro=#Pro"
Dim cmd1 As OleDb.OleDbCommand = New OleDbCommand(Query1, dbCon)
cmd1.Parameters.AddWithValue("#Pro", ComboBoxBP.SelectedItem.ToString)
Dim reader As OleDb.OleDbDataReader
reader = cmd1.ExecuteReader
While reader.Read()
TextBox1.Text = reader.GetValue(0).ToString
End While
reader.Close()
dbCon.Close()
End Using
I've tried using If reader.hasrows then display result in textbox, else show messagebox etc, but it doesn't work.
If reader.HasRows Then
While reader.Read()
TextBox1.Text = reader.GetValue(0).ToString
End While
Else
MessageBox.Show("asd")
End If
If I remove the .ToString from reader.GetValue(0) I get an error if the selected item from the combobox doesn't exist in the database. Cannot convert DBNull to integer or something.
So my question is, how to display a messagebox if the record "#Pro" doesn't exist?
Thanks~
Fixed(Workaround) with this
Dim ValueReturned As String
While reader.Read()
ValueReturned = reader.GetValue(0).ToString
If ValueReturned = "" Then
MessageBox.Show("Not Found", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
MetroTextBox1.Text = ValueReturned
End If
End While

Answer 1

Using OleDbDataReader is suboptimal for your query because it is never going to return a set of records to traverse:
Dim sql = "SELECT SUM(...=#p1"
' rather than sprinkling you connection string all over
' the app, you can make a function returning one
Using conn As OleDbConnection = GetConnection(),
cmd As New OleDbCommand(sql, GetConnection())
conn.Open())
' ToDo: Check that ComboBoxBP.SelectedItems.Count >0 before this
cmd.Parameters.AddWithValue("#p1", ComboBoxBP.SelectedItem.ToString)
' execute the query, get a result
Dim total = cmd.ExecuteScalar
' if there is no matches, OleDb returns DBNull
If total Is System.DBNull.Value Then
' no matches
MessageBox.Show("No matching records!"...)
Else
MessageBox.Show("The Total is: " & total.ToString()...)
End If
End Using ' disposes of the Connection and Command objects
Alteratively, you could use If IsDBNull(total) Then.... If you want, you can also convert it:
Dim total = cmd.ExecuteScalar.ToString()
' DBNull will convert to an empty string
If String.IsNullOrEmpty(total) Then
MessageBox.Show("No Soup For You!")
Else
...
End If

Answer 2

You could change your query to (this is for SQL-Server):
IF EXISTS (SELECT * FROM [T500] WHERE Pro = #Pro) SELECT SUM(Total) FROM [T500] WHERE Pro = #Pro ELSE SELECT 'Not Found'
And change your code to:
Dim ValueReturned As String
While reader.Read()
ValueReturned = reader.GetValue(0).ToString
End While
If ValueReturned Is Nothing OrElse ValueReturned = "Not Found" Then
MessageBox.Show("Not Found", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
Else
TextBox1.Text = ValueReturned
End If