LINQ Joins in C#

أنواع الـ LINQ Joins في C#

في الدرس ده هنتعرف على أنواع الـ Joins في LINQ، واللي بتسمح لنا نربط بين أكثر من مجموعة بيانات زي ما بنعمل في SQL.

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أنواع الـ Joins:

• Inner Join
• Left Outer Join
• Right Outer Join (محاكاة)
• Full Outer Join (محاكاة)
• Group Join
• Cross Join

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تعريف الكائنات:

public class Student
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public int StandardID { get; set; }
}

public class Standard
{
    public int StandardID { get; set; }
    public string StandardName { get; set; }
}

البيانات:

List<Student> studentList = new List<Student>()
{
    new Student() { StudentID = 1, StudentName = "John", StandardID = 1 },
    new Student() { StudentID = 2, StudentName = "Moin", StandardID = 1 },
    new Student() { StudentID = 3, StudentName = "Bill", StandardID = 2 },
    new Student() { StudentID = 4, StudentName = "Ram", StandardID = 2 },
    new Student() { StudentID = 5, StudentName = "Ron", StandardID = 3 }
};

List<Standard> standardList = new List<Standard>()
{
    new Standard(){ StandardID = 1, StandardName = "Standard 1"},
    new Standard(){ StandardID = 2, StandardName = "Standard 2"},
    new Standard(){ StandardID = 3, StandardName = "Standard 3"},
    new Standard(){ StandardID = 4, StandardName = "Standard 4"}
};

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1. Inner Join:

var innerJoin = from s in studentList
                join st in standardList
                on s.StandardID equals st.StandardID
                select new
                {
                    StudentName = s.StudentName,
                    StandardName = st.StandardName
                };

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2. Group Join:

var groupJoin = from std in standardList
                join s in studentList
                on std.StandardID equals s.StandardID into studentGroup
                select new
                {
                    StandardName = std.StandardName,
                    Students = studentGroup
                };

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3. Left Outer Join:

var leftJoin = from std in standardList
               join s in studentList
               on std.StandardID equals s.StandardID into studentGroup
               from student in studentGroup.DefaultIfEmpty()
               select new
               {
                   StandardName = std.StandardName,
                   StudentName = student?.StudentName ?? "No Student"
               };

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4. Cross Join:

var crossJoin = from std in standardList
                from s in studentList
                select new
                {
                    StudentName = s.StudentName,
                    StandardName = std.StandardName
                };

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ملحوظات:

• Join بيستخدم مفتاح مشترك للربط.
• GroupJoin مفيد لما نحتاج نربط عنصر رئيسي مع مجموعة.
• Left Join و Full Join بيتعملوا يدويًا عن طريق DefaultIfEmpty().