Question

I am stuck with rather confusing query.

Assume I have a ProductLending table that tracks what product each user has borrowed, when it was renewed, was it returned or not etc.. Given a user, I want to be able to select, all unique products that are still with the user.

table example:

userid    DateRenewed    ProductId    isReturned
````````````````````````````````````````````````
1        2011-07-21      15           0
1        2011-08-20      16           0
1        2011-09-21      15           1
2        2011-09-21      17           0
1        2011-09-21      15           0

This is a mock up so sorry if it’s not accurate.

Now, given userId = 1, I want to select just unique productId that are NOT returned, but are with the user. So this should give me 15, 16 as result, as even though 15 was returned, it was re-borrowed. If we delete the last row, then the result would just be 16, since user has only 16 with him.

I tried ordering by dateRenewed and selecting top 1 but it did totally something else.. how do I construct a query for this please?

Answer 1

If product is not returned by user, then the sum of bought products must be larger than sum of returned products

SELECT userid,ProductId FROM <table>
GROUP BY userid,ProductId HAVING SUM(CASE CAST(isReturned AS INT) WHEN 0 THEN 1 ELSE 0 END)-SUM(CAST(isReturned AS INT))>0