GROUP BY 相当于 Core Data [英] GROUP BY equivalent for Core Data

问题描述

我知道我可以使用@distinctUnionOfObjects 在 SQL 中查找类似以下内容的内容:

I know that I can use @distinctUnionOfObjects to find something like the following in SQL:

SELECT a_value
FROM my_table
GROUP BY a_value;

我正在寻找的是所有数据在一个数组中返回，而不仅仅是与 group by 表达式匹配的值数组.本质上，我正在寻找与以下核心数据的 SQL 查询等效的内容:

What I'm looking for is all of the data returned in an array, not just the array of values that matches the group by expression. Essentially, I'm looking for the equivalent of the following SQL query for core data:

SELECT *
FROM my_table
GROUP BY a_value;

推荐答案

It's analog

It's analog

SELECT 'Status', COUNT(*) FROM 'Records' GROUP BY 'Status':

NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:@"Record"];
NSEntityDescription* entity = [NSEntityDescription entityForName:@"Record"
                                          inManagedObjectContext:myManagedObjectContext];
NSAttributeDescription* statusDesc = [entity.attributesByName objectForKey:@"status"];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: @"url"]; // Does not really matter
NSExpression *countExpression = [NSExpression expressionForFunction: @"count:"
                                                          arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: @"count"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
[fetch setPropertiesToFetch:[NSArray arrayWithObjects:statusDesc, expressionDescription, nil]];
[fetch setPropertiesToGroupBy:[NSArray arrayWithObject:statusDesc]];
[fetch setResultType:NSDictionaryResultType];
NSError* error = nil;
NSArray *results = [myManagedObjectContext executeFetchRequest:fetch
                                                         error:&error];

在这里找到

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