Question

I have to retrieve only particular records whose sum value of size field is <=150.
I have table like below …

userid size
1       70
2      100   
3       50
4       25
5      120
6       90

The output should be …

userid size
1       70
3       50
4       25

For example, if we add 70,50,25 we get 145 which is <=150.

How would I write a query to accomplish this?

Answer 1

Here’s a query which will produce the above results:

SELECT * FROM `users` u
WHERE (select sum(size) from `users` where size <= u.size order by size) < 150
ORDER BY userid

However, the problem you describe of wanting the selection of users which would most closely fit into a given size, is a bin packing problem. This is an NP-Hard problem, and won’t be easily solved with ANSI SQL. However, the above seems to return the right result, but in fact it simply starts with the smallest item, and continues to add items until the bin is full.

A general, more effective bin packing algorithm would is to start with the largest item and continue to add smaller ones as they fit. This algorithm would select users 5 and 4.