核心数据函数类似于WHERE='(sql query)‘

Question

如何运行类似下面这样的sql query CoreData：

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

我读了NSPredicate指南和NSExpression，但不明白怎么做。

Answer 1

试试这个：

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

当耳机和old_headphones之间没有n对1的关系时，可以省略DISTINCT。

Answer 2

/*First request*/
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription 
                               entityForName:@"headphones" inManagedObjectContext:yourContext];
[fetchRequest setEntity:entity];

NSError *error;
NSArray *fetchedObjects = [yourContext executeFetchRequest:fetchRequest error:&error];

/*2nd request*/
NSFetchRequest *fetchRequest2 = [[NSFetchRequest alloc] init];
NSEntityDescription *entity2 = [NSEntityDescription 
                               entityForName:@"old_headphones" inManagedObjectContext:yourContext];
[fetchRequest setEntity:entity2];

NSError *error;
NSArray *fetchedObjects = [yourContext executeFetchRequest:fetchRequest2 error:&error];

//use your 2 NSArray to find your headphones