Ios 核心数据计数（或不同）

我有一张大约有100行的桌子。其中一列有一个名为“MeetingType”的列，该列始终是int值（1、2或3）

如何创建一个fetch谓词来返回不同类型行的计数

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例如，如果100行中有50行的类型为0，25行的类型为1，其余25行的类型为2。结果集的计数为3。（显示所有行中有3种不同的类型）

将结果作为一个集合获取，然后计算集合中的项目数

编辑#2：还有下面的链接可以帮助您获得更简洁的方法

编辑：好的，这里有一个尝试

//you will be working on some sort of entity, i've guessed its called Meeting
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Meeting" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

//you can set a predicate here if you like, eg only meetings with a certain name or something
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name = %@", type_name];
[request setPredicate:predicate];
// Specify that the request should return dictionaries.
[request setResultType:NSDictionaryResultType];

// Create an expression for the key path, in this case MeetingType
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"MeetingType"];


// Create an expression description using the maxExpression and returning a date.
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

// The name is the key that will be used in the dictionary for the return value.
[expressionDescription setName:@"meetingTypes"];
[expressionDescription setExpression:keyPathExpression];
[expressionDescription setExpressionResultType:NSInteger32AttributeType];

// Set the request's properties to fetch just the property represented by the expressions.
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

// Execute the fetch.
NSError *error = nil;
NSArray *objects = [sharedAISDataService.managedObjectContext executeFetchRequest:request error:&error];    

//check for no error
if(error == nil){
     //the following set will contain unique NSNumber objects
     NSSet *setOfMeetinTypes = [NSSet setWithArray:objects];

     int numberOfMeetinTypes = [setOfMeetinTypes count];
     //you now have the number of unique meeting types

}

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将结果作为一个集合获取，然后计算集合中的项目数

编辑#2：还有下面的链接可以帮助您获得更简洁的方法

编辑：好的，这里有一个尝试

//you will be working on some sort of entity, i've guessed its called Meeting
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Meeting" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

//you can set a predicate here if you like, eg only meetings with a certain name or something
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name = %@", type_name];
[request setPredicate:predicate];
// Specify that the request should return dictionaries.
[request setResultType:NSDictionaryResultType];

// Create an expression for the key path, in this case MeetingType
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"MeetingType"];


// Create an expression description using the maxExpression and returning a date.
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

// The name is the key that will be used in the dictionary for the return value.
[expressionDescription setName:@"meetingTypes"];
[expressionDescription setExpression:keyPathExpression];
[expressionDescription setExpressionResultType:NSInteger32AttributeType];

// Set the request's properties to fetch just the property represented by the expressions.
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

// Execute the fetch.
NSError *error = nil;
NSArray *objects = [sharedAISDataService.managedObjectContext executeFetchRequest:request error:&error];    

//check for no error
if(error == nil){
     //the following set will contain unique NSNumber objects
     NSSet *setOfMeetinTypes = [NSSet setWithArray:objects];

     int numberOfMeetinTypes = [setOfMeetinTypes count];
     //you now have the number of unique meeting types

}

---

除了在集合中计数之外，您还可以进行三次计数取回

NSError *error;
int count;
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([yourObject class])];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 1"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];

if (count > 0) {
    //Add 1 to your set or handle appropriately
}

fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 2"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (count > 0) {
    //Add 2 to your set or handle appropriately
}

fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 3"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (count > 0) {
    //Add 3 to your set or handle appropriately
}

---

这应该比实际拉出对象并在该集中搜索具有不同MeetingType属性的对象快得多。

除了在集合中计数外，还可以进行三次计数获取

NSError *error;
int count;
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([yourObject class])];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 1"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];

if (count > 0) {
    //Add 1 to your set or handle appropriately
}

fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 2"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (count > 0) {
    //Add 2 to your set or handle appropriately
}

fetchRequest.predicate = [NSPredicate predicateWithFormat:@"MeetingType == 3"];
count = [managedObjectContext countForFetchRequest:fetchRequest error:&error];
if (count > 0) {
    //Add 3 to your set or handle appropriately
}

---

这应该比实际拉出对象并在该集中搜索具有不同MeetingType属性的对象快得多。

这里是简单而纯粹的

NSFetchRequest

版本：

    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"Meeting"];
    [request setResultType:NSDictionaryResultType];
    [request setPropertiesToGroupBy:@[@"MeetingType"]];
    [request setPropertiesToFetch:@[@"MeetingType"]];
    NSError* error = nil;
    NSUInteger count = [[context executeFetchRequest:request error:&error] count];

---

以下是简单而纯粹的

NSFetchRequest

版本：

    NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"Meeting"];
    [request setResultType:NSDictionaryResultType];
    [request setPropertiesToGroupBy:@[@"MeetingType"]];
    [request setPropertiesToFetch:@[@"MeetingType"]];
    NSError* error = nil;
    NSUInteger count = [[context executeFetchRequest:request error:&error] count];

---

你能给我举个例子吗。我对核心数据，尤其是谓词是如此陌生，以至于我仍然难以从头开始创建它们。再次感谢。哦，我无法从我的头顶上获得语法正确的信息，但我应该能够给你足够的信息，让你走上正确的道路。你能给我举个例子吗。我对核心数据，尤其是谓词是如此陌生，以至于我仍然难以从头开始创建它们。再次感谢。哦，我无法从我的头顶上获得语法正确的信息，但我应该能够给你足够的信息，让你走上正确的道路。