Question

How can run sql query CoreData similar this:

SELECT COUNT(*) FROM headphones WHERE id IN (SELECT id FROM old_headphones);

I read NSPredicate guide and NSExpression, but don’t understand how to do this.

Answer 1

Try this:

SELECT COUNT(distinct headphones.Id) 
FROM headphones 
    INNER JOIN old_headphones ON
        headphones.Id = old_headphones.Id

When there is not an n-to_1 relation between headphones and old_headphones, you can omit the DISTINCT.