How To Delete Records With A Field Containing NULL

Hi,

 All records of a table must be deleted if a field (field1) contains NULL or is made empty by a previous update command.

I did this but it only works when the field is made empty, not when it contains NULL:

dim field1 as string = ""
...
sql = "delete from mytable WHERE (field1=@field1)"
        comd = New SqlCommand(sql, oConnection)
        comd.Parameters.AddWithValue("@field1", field1)
        comd.ExecuteNonQuery() 

How can i do that?

Thanks

Tartuffe