I am using MySQL and PHP. I have a table that contains the columns id and quantity. I would like to retrieve the id of the row that is the last to sum quantity as it reaches the number 40 at sum. To be more specific. I have 3 rows in the database. One with quantity 10, one with quantity 30 and one with quantity 20. So if I sum the quantities to have the result 40, I would sum up the first two witch means: 10 + 30 = 40. That way, the last Id that is used to sum the number 40 is 2. I just want to know the id of the last row that is used to complete the sum of 40.

I would give further details if asked. THANK YOU!!

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Let me put it this way:
I really have 6 products in my hand. The first one came to my possession on the date of 10, the next 3 came on the date of 11 and the last 2 came on 12. 

Now, I want to sell 3 products from my stock. And want to sell them in the order that they came. So for the customer that wants 3 products I would sell him the product that came on 10 and 2 products from the ones that came on 11. 

For the next customer that wants 2 products, I would sell him one product from the date of 11 that remains from the last order of 3 products, and another one from the ones on 12.

The question is how would I know which price had each product I sold ? I thought that if I can find out which rows sums up every requested quantity, I would know where to start the sum every time I want to deliver an order. So first I would look which rows sums up 3 products and keep the entry id. For the next order I would start the count from that ID and sum until it sums up the second order of 2 products and so on. I thought that this way, I can keep track of the incoming prices that each product had. So I won't sell the products from the date of 12 at a price made up using the first prices.

I hope you understand. I just need to know what price had any of my products so I would know that the first products would have one price but as the product prices raises, I must raise my prices too...So the last products that came must be sold for a higher price. I can only achieve that if I keep track of this...

Thank you very much.

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Nobody ? Or, even easier: MySQL should select the needed rows for SUM(`quantity`) to be higher or equal with 40 for example. And then to get me the id of the last row that participated at the sum process.