SQL SELECT with table aliases in Core Data - ios

Question

I have the following SQL query that I want to do using Core Data:
SELECT t1.date, t1.amount + SUM(t2.amount) AS importantvalue
FROM specifictable AS t1, specifictable AS t2
WHERE t1.amount < 0 AND t2.amount < 0 AND t1.date IS NOT NULL AND t2.date IS NULL
GROUP BY t1.date, t1.amount;
Now, it looks like CoreData fetch requests can only fetch from a single entity. Is there a way to do this entire query in a single fetch request?

Answer 1

The best way I know is to crate an abstract parent entity for entities you wish to fetch together.
So if you have - 'Meat' 'Vegetables' and 'Fruits' entities, you can create a parent abstract entity for 'Food' and then fetch for all the sweet entities in the 'Food' entity.
This way you will get all the sweet 'Meat' 'Vegetables' and 'Fruits'.
Look here:
Entity Inheritance in Apple documentation.

Answer 2

Nikolay,
Core Data is not a SQL system. It has a more primitive query language. While this appears to be a deficit, it really isn't. It forces you to bring things into RAM and do your complex calculations there instead of in the DB. The NSSet/NSMutableSet operations are extremely fast and effective. This also results in a faster app. (This is particularly apparent on iOS where the flash is slow and, hence, big fetches are to be preferred.)
In answer to your question, yes, a fetch request operates on a single entity. No, you are not limited to data on that entity. One uses key paths to traverse relationships in the predicate language.
Shannoga's answer is one good way to solve your problem. But I don't know enough about what you are actually trying to accomplish with your data model to judge whether using entity inheritance is the right path for your app. It may not be.
Your SQL schema from a server may not make sense in a CD app. Both the query language and how the data is used in the UI probably force a different structure. (For example, using a fetched results controller on iOS can force you to denormalize your data differently than you would on a server.)
Entity inheritance, like inheritance in OOP, is a stiff technology. It is hard to change. Hence, I use it carefully. When I do use it, I gain performance in some fetches and some simplification in other calculations. At other times, it is the wrong answer, performance wise.
The real answer is a question: what are you really trying to do?
Andrew