insert into factclientes (IDFactClientes, IDTercero, SubTotal, IVA, Total)
        values ('', '3', '2500.00', '400.00', '2900.00')
        ON DUPLICATE KEY UPDATE IDTercero = values(IDTercero),
                SubTotal = values(SubTotal),
                IVA = values(IVA),
                Total = values(Total)

Incorrect integer value: '' for column 'IDFactClientes' at row 1