数据库查询 返回 符合条件的属性的总数或平均值等
let fetchRequest = NSFetchRequest(entityName: "Venue")
fetchRequest.resultType = .DictionaryResultType
let sumExpressionDesc = NSExpressionDescription()
//添加key方便从fetchrequest获取数据
sumExpressionDesc.name = "sumDeals"
//sum: 为NSExpression 默认函数 计算总数
//specialCount 数据库对象 Venue 的一个属性
sumExpressionDesc.expression = NSExpression(forFunction: "sum:", arguments: [NSExpression(forKeyPath: "specialCount")])
sumExpressionDesc.expressionResultType = .Integer32AttributeType
fetchRequest.propertiesToFetch = [sumExpressionDesc]
do {
let results =
try coreDataStack.context.executeFetchRequest(fetchRequest) as! [NSDictionary]
let resultDict = results.first!
let numDeals = resultDict["sumDeals"]
numDealsLabel.text = "\(numDeals!) total deals"
} catch let error as NSError {
print("Could not fetch \(error), \(error.userInfo)")
}
NSExpression()方法添加 操作函数名 默认支持的函数可查看此类
使用fetchRequest.propertiesToFetch 查询数据库
