Postgres 12

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create table product as 
    select generate_series(1, 10, 1) as id, random() as quantity
10 rows affected

SELECT id FROM product WHERE quantity >= ALL(SELECT quantity FROM product)

id

4

SELECT id FROM product WHERE quantity = (SELECT MAX(quantity) FROM product)

id

4

insert into product (id, quantity) values (11, null)
1 rows affected

SELECT id FROM product WHERE quantity >= ALL(SELECT quantity FROM product)
SELECT id FROM product WHERE quantity = (SELECT MAX(quantity) FROM product)

id

4