如何在SWIFT中的IOS CORE-DATA请求中使用SQL GROUP BY和SUM函数？

SELECT Account_name,SUM(Amount) 
    FROM Transactions
    WHERE Account_name like 'Private%'
    and Amount > 1000
    GROUP BY Account_name
    ORDER BY Account_name DESC

请记住,CoreData不是关系数据库,因此您应该认为实体不是“表”,对象不是“记录”.另请注意,按照惯例,属性名称不应以大写字母开头.也就是说,可以构造一个fetch来实现你想要的.关键步骤是：

>创建一个获取请求
>根据您选择的标准指定nspredicate进行过滤
>将resultType设置为.DictionaryResultType(“分组依据”必需)
>设置要包含在fetch中的属性(获取sum(),这涉及创建NSExpression和关联的NSExpressionDescription).
>将属性设置为分组依据
>创建一个NSSortDescriptor按名称降序排序
>执行提取

请参阅以下代码：

let fetch = NSFetchRequest(entityName: "Transaction")
let predicate = nspredicate(format: "Account_name like %@ AND Amount > %@","Private*",NSNumber(double: 1000.0))
fetch.predicate = predicate
fetch.resultType = .DictionaryResultType
let sumExpression = NSExpression(format: "sum:(Amount)")
let sumED = NSExpressionDescription()
sumED.expression = sumExpression
sumED.name = "sumOfAmount"
sumED.expressionResultType = .DoubleAttributeType
fetch.propertiesToFetch = ["Account_name",sumED]
fetch.propertiesToGroupBy = ["Account_name"]
let sort = NSSortDescriptor(key: "Account_name",ascending: false)
fetch.sortDescriptors = [sort]
let results = managedobjectContext?.executeFetchRequest(fetch,error: nil) as NSArray?

结果将是一个字典数组,其键为“Account_name”和“sumOfAmount”.

编辑
要提取特定Account_name的值,请使用另一个谓词：

let newPredicate = nspredicate(format: "Account_name like %@","toto")!
    if let resultsArray = results { // to unwrap the optional
        let filteredResults = resultsArray.filteredArrayUsingPredicate(newPredicate)
        let sumOfAmount = (filteredResults[0] as NSDictionary)["sumOfAmount"] as Double
    }

我忽略了前一个示例将Account_name过滤为以“Private”开头的事实,并假设您将拥有一个(且只有一个)名称为“toto”的帐户(因此filteredResults [0]);你的生产代码应该检查.