php - Export big mysql table into multiple small csv/excel file

I have a big table of around 1000000 rows with 3 colums i.e id(primary key),t1 and t2. I am displaying that table on front end using bootstrap datatable. I can export that table using datatable function like

    $('#example').DataTable({
    dom: 'Bfrtip',
    buttons: [
        'copy', 'csv', 'excel', 'pdf', 'print'
    ]
    });

but i want csv/excel file into multiple chunks. How should i do it