Question

I am connecting to a MS-Access 2007 database using VB 2010 and OLEDB. Conducting the following test seems to suggest that MS-Access does not interpret DBNull.Value correctly when sent as a parameter from OLEDB:

(The Hospital table contains 1 row with the “LatLong” column set to null)

Dim cnt = Common.GetScalar(axsCon, "SELECT Count(*) FROM Hospitals WHERE LatLong = @LL ", _
                New String() {"LL"},
                New Object() {DBNull.Value})

This query returns cnt = 0

However:
cnt = Common.GetScalar(axsCon, “SELECT Count(*) FROM Hospitals WHERE LatLong IS NULL “)

returns cnt = 1

Any ideas are appreciated.

p.s.:
Common.GetScalar looks like:

Public Shared Function GetScalar( _
    ByRef OleCon As OleDbConnection, _
    ByRef SQL As String, _
    Optional ByRef Params() As String = Nothing, _
    Optional ByRef Vals() As Object = Nothing, _
    Optional IsQuery As Boolean = False) As Object
    Try
        Dim oleCmd As OleDbCommand = OleCon.CreateCommand
        oleCmd.CommandType = IIf(IsQuery, CommandType.StoredProcedure, CommandType.Text)
        oleCmd.CommandText = SQL
        If Not Params Is Nothing Then
            Dim pInx As Int16
            For pInx = 0 To Params.Count - 1
                oleCmd.Parameters.AddWithValue(Params(pInx), Vals(pInx))
            Next
        End If
        Return oleCmd.ExecuteScalar()
    Catch ex As Exception
        Throw New Exception(ex.Message)
    End Try
End Function

TIA

Answer 1

Change

"SELECT Count(*) FROM Hospitals WHERE LatLong = @LL"

to

“SELECT Count(*)
FROM Hospitals
WHERE 1=
    CASE
        WHEN @LL IS null AND LatLong IS null THEN 1
        WHEN LatLong = @LL THEN 1
    END”

This will then check for null or matching value. Nulls can be very very tricky.