Question

I am trying to retrieve a maximum value from sql – vb.net

This is my code: I get this error:

Unable to cast object of type ‘System.DBNull’ to type
‘Oracle.DataAccess.Client.OracleDataReader’

Please help. 🙁

Dim cmd2 As New OracleCommand
cmd2.Connection = conn
'SELECT MAX(LASTNO) FROM d001005 where lbrcode = '104' and CODE1 = 'CASH' and lnodate = '14-NOV-08' 
Dim datepara As String = Date.Now.ToString("dd-MMM-yy")
'datepara = "14-NOV-08"
cmd2.CommandText = "SELECT MAX(LASTNO) FROM d001005 where lbrcode = '" + lbr + "' and CODE1 = 'CASH' and lnodate = '" + datepara + "'"
cmd2.CommandType = CommandType.Text
Dim dr2 As OracleDataReader = cmd2.ExecuteScalar
Dim result As Decimal = dr2.Item("LASTNO")

Answer 1

There is no item with the name “LASTNO” as you are using an aggregate function to find out the maximum value. Furthermore ExecuteScalar returns the first value of the first row! Another point is, that you should use OracleParameter for any kind of dynamic statement, to prevent SQL injection!

Dim cmd2 As New OracleCommand
cmd2.Connection = conn
Dim datepara As String = Date.Now.ToString("dd-MMM-yy") ' see hint below

cmd2.CommandText = "SELECT MAX(LASTNO) as maxLastNo FROM d001005 where lbrcode = :lbr and CODE1 = 'CASH' and lnodate = :lnodate"

' add parameter value directly
cmd2.Parameters.AddWithValue("lbr", lbr)   ' lbr seems to be your var which holds the according value

' or create parameter object and set type and add it afterwards
Dim param as new OracleParameter
param.ParameterName = "lnodate"
param.Value = datepara    ' or any other value (furthermore: no need to convert to string)
param.ParameterType = OracleType.DateTime ' or OracleType.Varchar which is set by default
cmd2.CommandType = CommandType.Text

Dim val = cmd2.ExecuteScalar()
if Not val is Nothing then
    Dim maxValue as Decimal = Convert.ToDecimal(val)
    Console.WriteLine(maxValue)
End if