I have a problem and I can't solve it.

Its about designing a database with table where items are stored, but those items can be with more than one selling price - two and more, or those items can be with more than one expiring date.

For example, one article once can come with 20 dollars price, 30 pieces. After some days, the same article can come with 30 dollars price, 10 pieces. BUT, the problem is that there are still articles for 20 dollars!