クエリ結果のグループ化Group query results

グループ化は、LINQ の最も強力な機能の 1 つです。Grouping is one of the most powerful capabilities of LINQ. 次の例では、さまざまな方法でデータをグループ化する方法を示します。The following examples show how to group data in various ways:

  • 1 つのプロパティで。By a single property.

  • 文字列プロパティの最初の文字で。By the first letter of a string property.

  • 計算された数値の範囲で。By a computed numeric range.

  • ブール述語またはその他の式で。By Boolean predicate or other expression.

  • 複合キーで。By a compound key.

さらに、最後の 2 つのクエリは、学生の名と姓だけを含む新しい匿名型に結果を射影します。In addition, the last two queries project their results into a new anonymous type that contains only the student's first and last name. 詳しくは、「group 句」をご覧ください。For more information, see the group clause.

例Example

このトピックのすべての例では、次のヘルパー クラスとデータ ソースを使います。All the examples in this topic use the following helper classes and data sources.

public class StudentClass
{
    #region data
    protected enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };
    protected class Student
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int ID { get; set; }
        public GradeLevel Year;
        public List<int> ExamScores;
    }

    protected static List<Student> students = new List<Student>
    {
        new Student {FirstName = "Terry", LastName = "Adams", ID = 120, 
            Year = GradeLevel.SecondYear, 
            ExamScores = new List<int>{ 99, 82, 81, 79}},
        new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116, 
            Year = GradeLevel.ThirdYear,
            ExamScores = new List<int>{ 99, 86, 90, 94}},
        new Student {FirstName = "Hanying", LastName = "Feng", ID = 117, 
            Year = GradeLevel.FirstYear, 
            ExamScores = new List<int>{ 93, 92, 80, 87}},
        new Student {FirstName = "Cesar", LastName = "Garcia", ID = 114, 
            Year = GradeLevel.FourthYear,
            ExamScores = new List<int>{ 97, 89, 85, 82}},
        new Student {FirstName = "Debra", LastName = "Garcia", ID = 115, 
            Year = GradeLevel.ThirdYear, 
            ExamScores = new List<int>{ 35, 72, 91, 70}},
        new Student {FirstName = "Hugo", LastName = "Garcia", ID = 118, 
            Year = GradeLevel.SecondYear, 
            ExamScores = new List<int>{ 92, 90, 83, 78}},
        new Student {FirstName = "Sven", LastName = "Mortensen", ID = 113, 
            Year = GradeLevel.FirstYear, 
            ExamScores = new List<int>{ 88, 94, 65, 91}},
        new Student {FirstName = "Claire", LastName = "O'Donnell", ID = 112, 
            Year = GradeLevel.FourthYear, 
            ExamScores = new List<int>{ 75, 84, 91, 39}},
        new Student {FirstName = "Svetlana", LastName = "Omelchenko", ID = 111, 
            Year = GradeLevel.SecondYear, 
            ExamScores = new List<int>{ 97, 92, 81, 60}},
        new Student {FirstName = "Lance", LastName = "Tucker", ID = 119, 
            Year = GradeLevel.ThirdYear, 
            ExamScores = new List<int>{ 68, 79, 88, 92}},
        new Student {FirstName = "Michael", LastName = "Tucker", ID = 122, 
            Year = GradeLevel.FirstYear, 
            ExamScores = new List<int>{ 94, 92, 91, 91}},
        new Student {FirstName = "Eugene", LastName = "Zabokritski", ID = 121,
            Year = GradeLevel.FourthYear, 
            ExamScores = new List<int>{ 96, 85, 91, 60}}
    };
    #endregion

    //Helper method, used in GroupByRange.
    protected static int GetPercentile(Student s)
    {
        double avg = s.ExamScores.Average();
        return avg > 0 ? (int)avg / 10 : 0;
    }

    

    public void QueryHighScores(int exam, int score)
    {
        var highScores = from student in students
                         where student.ExamScores[exam] > score
                         select new {Name = student.FirstName, Score = student.ExamScores[exam]};

        foreach (var item in highScores)
        {
            Console.WriteLine($"{item.Name,-15}{item.Score}");
        }
    }
}

public class Program
{
    public static void Main()
    {
        StudentClass sc = new StudentClass();
        sc.QueryHighScores(1, 90);

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit");
        Console.ReadKey();
    }
}

例Example

次の例では、要素の 1 つのプロパティをグループ化キーとして使って、ソース要素をグループ化する方法を示します。The following example shows how to group source elements by using a single property of the element as the group key. この場合、キーは学生の姓である string です。In this case the key is a string, the student's last name. また、キーの部分文字列を使うこともできます。It is also possible to use a substring for the key. グループ化操作では、型の既定の等値比較子を使います。The grouping operation uses the default equality comparer for the type.

次のメソッドを StudentClass クラスに貼り付けます。Paste the following method into the StudentClass class. Main メソッドの呼び出しステートメントを sc.GroupBySingleProperty() に変更します。Change the calling statement in the Main method to sc.GroupBySingleProperty().

public void GroupBySingleProperty()
{
    Console.WriteLine("Group by a single property in an object:");

    // Variable queryLastNames is an IEnumerable<IGrouping<string, 
    // DataClass.Student>>. 
    var queryLastNames =
        from student in students
        group student by student.LastName into newGroup
        orderby newGroup.Key
        select newGroup;

    foreach (var nameGroup in queryLastNames)
    {
        Console.WriteLine($"Key: {nameGroup.Key}");
        foreach (var student in nameGroup)
        {
            Console.WriteLine($"\t{student.LastName}, {student.FirstName}");
        }
    }
}
/* Output:
    Group by a single property in an object:
    Key: Adams
            Adams, Terry
    Key: Fakhouri
            Fakhouri, Fadi
    Key: Feng
            Feng, Hanying
    Key: Garcia
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: Mortensen
            Mortensen, Sven
    Key: O'Donnell
            O'Donnell, Claire
    Key: Omelchenko
            Omelchenko, Svetlana
    Key: Tucker
            Tucker, Lance
            Tucker, Michael
    Key: Zabokritski
            Zabokritski, Eugene
*/

例Example

次の例では、オブジェクトのプロパティ以外の何かをグループ化キーとして使って、ソース要素をグループ化する方法を示します。The following example shows how to group source elements by using something other than a property of the object for the group key. この例では、キーは学生の姓の最初の文字です。In this example, the key is the first letter of the student's last name.

次のメソッドを StudentClass クラスに貼り付けます。Paste the following method into the StudentClass class. Main メソッドの呼び出しステートメントを sc.GroupBySubstring() に変更します。Change the calling statement in the Main method to sc.GroupBySubstring().

public void GroupBySubstring()
{            
    Console.WriteLine("\r\nGroup by something other than a property of the object:");

    var queryFirstLetters =
        from student in students
        group student by student.LastName[0];

    foreach (var studentGroup in queryFirstLetters)
    {
        Console.WriteLine($"Key: {studentGroup.Key}");
        // Nested foreach is required to access group items.
        foreach (var student in studentGroup)
        {
            Console.WriteLine($"\t{student.LastName}, {student.FirstName}");
        }
    }           
}
/* Output:
    Group by something other than a property of the object:
    Key: A
            Adams, Terry
    Key: F
            Fakhouri, Fadi
            Feng, Hanying
    Key: G
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: M
            Mortensen, Sven
    Key: O
            O'Donnell, Claire
            Omelchenko, Svetlana
    Key: T
            Tucker, Lance
            Tucker, Michael
    Key: Z
            Zabokritski, Eugene
*/

例Example

次の例では、数値範囲をグループ化キーとして使って、ソース要素をグループ化する方法を示します。The following example shows how to group source elements by using a numeric range as a group key. クエリは、名と姓および学生が属しているパーセンタイル範囲のみを含む匿名型に、結果を投影します。The query then projects the results into an anonymous type that contains only the first and last name and the percentile range to which the student belongs. 匿名型を使っているのは、結果を表示するために完全な Student オブジェクトを使う必要がないためです。An anonymous type is used because it is not necessary to use the complete Student object to display the results. GetPercentile は、学生の平均スコアに基づいてパーセンタイルを計算するヘルパー関数です。GetPercentile is a helper function that calculates a percentile based on the student's average score. メソッドは、0 から 10 の間の整数を返します。The method returns an integer between 0 and 10.

//Helper method, used in GroupByRange.
protected static int GetPercentile(Student s)
{
    double avg = s.ExamScores.Average();
    return avg > 0 ? (int)avg / 10 : 0;
}

次のメソッドを StudentClass クラスに貼り付けます。Paste the following method into the StudentClass class. Main メソッドの呼び出しステートメントを sc.GroupByRange() に変更します。Change the calling statement in the Main method to sc.GroupByRange().

public void GroupByRange()
{            
    Console.WriteLine("\r\nGroup by numeric range and project into a new anonymous type:");

    var queryNumericRange =
        from student in students
        let percentile = GetPercentile(student)
        group new { student.FirstName, student.LastName } by percentile into percentGroup
        orderby percentGroup.Key
        select percentGroup;

    // Nested foreach required to iterate over groups and group items.
    foreach (var studentGroup in queryNumericRange)
    {
        Console.WriteLine($"Key: {studentGroup.Key * 10}"); 
        foreach (var item in studentGroup)
        {
            Console.WriteLine($"\t{item.LastName}, {item.FirstName}");
        }
    }            
}
/* Output:
    Group by numeric range and project into a new anonymous type:
    Key: 60
            Garcia, Debra
    Key: 70
            O'Donnell, Claire
    Key: 80
            Adams, Terry
            Feng, Hanying
            Garcia, Cesar
            Garcia, Hugo
            Mortensen, Sven
            Omelchenko, Svetlana
            Tucker, Lance
            Zabokritski, Eugene
    Key: 90
            Fakhouri, Fadi
            Tucker, Michael
*/

例Example

次の例では、ブール比較式を使って、ソース要素をグループ化する方法を示します。The following example shows how to group source elements by using a Boolean comparison expression. この例のブール式は、学生の平均試験スコアが 75 より大きいかどうかをテストします。In this example, the Boolean expression tests whether a student's average exam score is greater than 75. 前の例と同じように、完全なソース要素が必要ないため、結果を匿名型に投影します。As in previous examples, the results are projected into an anonymous type because the complete source element is not needed. 匿名型のプロパティは Key メンバーのプロパティになり、クエリ実行時に名前でアクセスできることに注意してください。Note that the properties in the anonymous type become properties on the Key member and can be accessed by name when the query is executed.

次のメソッドを StudentClass クラスに貼り付けます。Paste the following method into the StudentClass class. Main メソッドの呼び出しステートメントを sc.GroupByBoolean() に変更します。Change the calling statement in the Main method to sc.GroupByBoolean().

public void GroupByBoolean()
{            
    Console.WriteLine("\r\nGroup by a Boolean into two groups with string keys");
    Console.WriteLine("\"True\" and \"False\" and project into a new anonymous type:");
    var queryGroupByAverages = from student in students
                               group new { student.FirstName, student.LastName }
                                    by student.ExamScores.Average() > 75 into studentGroup
                               select studentGroup;

    foreach (var studentGroup in queryGroupByAverages)
    {
        Console.WriteLine($"Key: {studentGroup.Key}");
        foreach (var student in studentGroup)
            Console.WriteLine($"\t{student.FirstName} {student.LastName}");
    }            
}
/* Output:
    Group by a Boolean into two groups with string keys
    "True" and "False" and project into a new anonymous type:
    Key: True
            Terry Adams
            Fadi Fakhouri
            Hanying Feng
            Cesar Garcia
            Hugo Garcia
            Sven Mortensen
            Svetlana Omelchenko
            Lance Tucker
            Michael Tucker
            Eugene Zabokritski
    Key: False
            Debra Garcia
            Claire O'Donnell
*/

例Example

次の例では、匿名型を使って、複数の値を含むキーをカプセル化する方法を示します。The following example shows how to use an anonymous type to encapsulate a key that contains multiple values. この例では、最初のキーの値は学生の姓の最初の文字です。In this example, the first key value is the first letter of the student's last name. 2 番目のキーの値は、最初の試験での学生のスコアが 85 より高いかどうかを示すブール値です。The second key value is a Boolean that specifies whether the student scored over 85 on the first exam. キーの任意のプロパティでグループを並べ替えることができます。You can order the groups by any property in the key.

次のメソッドを StudentClass クラスに貼り付けます。Paste the following method into the StudentClass class. Main メソッドの呼び出しステートメントを sc.GroupByCompositeKey() に変更します。Change the calling statement in the Main method to sc.GroupByCompositeKey().

public void GroupByCompositeKey()
{
    var queryHighScoreGroups =
        from student in students
        group student by new { FirstLetter = student.LastName[0], 
            Score = student.ExamScores[0] > 85 } into studentGroup
        orderby studentGroup.Key.FirstLetter
        select studentGroup;

    Console.WriteLine("\r\nGroup and order by a compound key:");
    foreach (var scoreGroup in queryHighScoreGroups)
    {
        string s = scoreGroup.Key.Score == true ? "more than" : "less than";
        Console.WriteLine($"Name starts with {scoreGroup.Key.FirstLetter} who scored {s} 85");
        foreach (var item in scoreGroup)
        {
            Console.WriteLine($"\t{item.FirstName} {item.LastName}");
        }
    }
}

/* Output:
    Group and order by a compound key:
    Name starts with A who scored more than 85
            Terry Adams
    Name starts with F who scored more than 85
            Fadi Fakhouri
            Hanying Feng
    Name starts with G who scored more than 85
            Cesar Garcia
            Hugo Garcia
    Name starts with G who scored less than 85
            Debra Garcia
    Name starts with M who scored more than 85
            Sven Mortensen
    Name starts with O who scored less than 85
            Claire O'Donnell
    Name starts with O who scored more than 85
            Svetlana Omelchenko
    Name starts with T who scored less than 85
            Lance Tucker
    Name starts with T who scored more than 85
            Michael Tucker
    Name starts with Z who scored more than 85
            Eugene Zabokritski
*/

関連項目See also