MySQL customers invoices

SELECT DISTINCT
	invoice.invoice_id,
	invoice.invoice_total,
	invoice.invoice_paid,
	invoice.invoice_tax,
	invoice.ts_printed,
	invoice.ts_add,
	ROUND(invoice.invoice_total-invoice.invoice_paid, 2) AS to_pay,
	orders.customer_ref,
	delivery_note.delivery_note_id
	FROM
	invoice
	INNER JOIN order_items ON order_items.invoice_ref = invoice.invoice_id
	INNER JOIN orders ON order_items.order_ref = orders.order_id
	INNER JOIN delivery_note ON order_items.delivery_note_ref = delivery_note.delivery_note_id
	WHERE
	orders.customer_ref = '$cid'
	AND
	invoice.invoice_paid < invoice.invoice_total
	ORDER BY
	invoice.invoice_id DESC