使用Core Data计算一个简单的和 [英] Calculating a simple sum with Core Data

问题描述

我有一个简单的Core Data存储，其实体Cost具有整数属性value。我想汇总我的商店中的所有成本，这相当于下面的sql语句：

I have a simple Core Data store with an entity Cost with an integer property "value". I want to sum up all amounts of the costs in my store, which is equivalent to the following sql statement:

SELECT sum(value) FROM costs

如何在Cocoa Touch中以最有效的方式使用Core Data？

How do I do it the most efficient way in Cocoa Touch? By using Core Data? Or just get all cost entities and do the summation manually?

推荐答案

最好的方法是使用获取特定值和供应a 具有 sum：函数的NSExpressionDescription。

The best way is to use a fetch for specific values and supply a NSExpressionDescription with a sum: function.

执行提取时，您将获得一个包含字典的一个元素数组，其字符串与表达式描述匹配，其值是表达式的结果。在这种情况下，您会得到一个 sum 键，其值将是给定表达式的属性的总和。

When you execute the fetch you get a one element array containing a dictionary whose keys match the expression descriptions and whose values are the results of the expressions. In this case, you would get a sum key whose value would be the sum of the attributes given the expression.

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