Core Data - How to Do a SELECT DISTINCT

翻译于 2015-01-12 16:02:12 发布 · 627 阅读

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本文介绍如何使用 Core Data 进行不重复数据查询，并通过 NSFetchRequest 和 NSEntityDescription 实现对 People 实体中 age 属性的不重复查询。文章还展示了如何设置排序和获取查询结果。

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NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"People" inManagedObjectContext:managedObjectContext];
request.entity = entity;
request.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:@"age"]];
request.returnsDistinctResults = YES;
request.resultType = NSDictionaryResultType;//必须,如果没有这一句,上边限定不重复的语句就被忽略

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"age" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptors]];
[sortDescriptor release];

NSError *error = nil;
NSArray *distincResults = [managedObjectContext executeFetchRequest:request error:&error];
// Use the results
[request release];

源: http://felipecypriano.com/2011/09/21/core-data-how-to-do-a-select-distinct/