null parameter in asp or ado command object
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I have a form in asp with two text boxes. If user enters a value in it and submit, it displays data from MS access query for the selected dates by user. If users just leave both text boxes blanks, I want to display complete output from the database.  
    This is my sample query  in access :

    select column_date, field1, field2, sum(field3) from table1
    where field1 like '*xyz' and 
    column_date between [@startdate] and [@enddate]
    group by column_date, field1, field2


My asp codes are similar to below :

    objCmd.CommandText = "Query"
    objCmd.CommandType = adCmdStoredProc
    Set objParam = objCmd.CreateParameter("@startdate" , adInteger, adParamInput, 0, 0)
    objCmd.Parameters.Append objParam
    Set objParam = objCmd.CreateParameter("@enddate" , adInteger, adParamInput, 0, 0)
    objCmd.Parameters.Append objParam
    objCmd.Parameters("@startdate") = request.form("startdate")
    objCmd.Parameters("@enddate") = request.form("enddate")
    ................
    
Please note my startdate and enddates are text datatype with format mm-dd-yyyy.

I have two text boxes name "startdate" and "enddate". When user enters dates in the boxes, it returns data between the two dates from the query. If user leaves blank, it shows error.

But I want to make sure if user leave both text boxes blank, it returns all values from the query.  If user input a single value in any of the two text boxes, it should return data only for that date. 

I'm not sure how can I achieve it. 

    