iPhone：Core Data，如何从db获取值的总和 [英] iPhone: Core Data, How to get sum of values from db

问题描述

我有一个模型笔，它有这样的属性：makerName和计数。如何获得所有的计数的总和？ Core Data能为我计算吗？感谢...

I have a model 'Pens' and it has such properties: "makerName" and "Count". How can I get sum of all 'Count' ? Can Core Data count that for me ? Thanks...

推荐答案

有两种方法可以解决这个问题：

first - 到NSSet或NSArray并使用@sum运算符：

There are two ways to solve this problem:
first - get your data to NSSet or NSArray and use @sum operator:

//assume that `pens` are NSArray of Pen
NSNumber *countSum=[pens valueForKeyPath:"@sum.count"];

其次是使用特定撷取具体值和添加的 NSExpressionDescription 与一个总和。这种方式更难，但对于更大的数据库

second is using specific fetch for specific value with added NSExpressionDescription with a sum. This way is harder but better for larger db's

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