Det är inte vackert, men jag tror att det här gör det och kanske kan det vara grunden till något mindre krångligt. Observera att jag använder en "falsk" INNER JOIN bara för att få någon variabel initialiserad för första gången - den har ingen annan roll.
SELECT ID,
supplier,
qty,
cumulative_qty
FROM
(
SELECT
ID,
supplier,
qty,
-- next line keeps a running total quantity by supplier id
@cumulative_quantity := if (@sup <> supplier, qty, @cumulative_quantity + qty) as cumulative_qty,
-- next is 0 for running total < 5 by supplier, 1 the first time >= 5, and ++ after
@reached_five := if (@cumulative_quantity < 5, 0, if (@sup <> supplier, 1, @reached_five + 1)) as reached_five,
-- next takes note of changes in supplier being processed
@sup := if(@sup <> supplier, supplier, @sup) as sup
FROM
(
--this subquery is key for getting things in supplier order, by descending id
SELECT *
FROM `sample_table`
ORDER BY supplier, ID DESC
) reverse_order_by_id
INNER JOIN
(
-- initialize the variables used to their first ever values
SELECT @cumulative_quantity := 0, @sup := 0, @reached_five := 0
) only_here_to_initialize_variables
) t_alias
where reached_five <= 1 -- only get things up through the time we first get to 5 or above.