LINQ Left Outer Join

LINQ Left Outer Join

في هذا الدرس، هنتعلّم إزاي ننفّذ Left Outer Join باستخدام LINQ في C#، سواء بـ Query Syntax أو Method Syntax.

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ما هو Left Outer Join؟

بيرجع كل العناصر من المجموعة اليسرى (زي الأقسام)، سواء وُجد لها عنصر مطابق في المجموعة اليمنى (زي الموظفين) أو لأ. ولو مفيش، بيرجع null.

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تعريف الكائنات:

public class Employee
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int DepartmentID { get; set; }
}

public class Department
{
    public int ID { get; set; }
    public string DeptName { get; set; }
}

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البيانات:

List<Employee> employees = new List<Employee>()
{
    new Employee() { ID = 1, Name = "John", DepartmentID = 1 },
    new Employee() { ID = 2, Name = "Moin", DepartmentID = 1 },
    new Employee() { ID = 3, Name = "Bill", DepartmentID = 2 },
    new Employee() { ID = 4, Name = "Ram", DepartmentID = 2 },
    new Employee() { ID = 5, Name = "Ron", DepartmentID = 3 }
};

List<Department> departments = new List<Department>()
{
    new Department(){ ID = 1, DeptName = "IT"},
    new Department(){ ID = 2, DeptName = "HR"},
    new Department(){ ID = 3, DeptName = "Finance"},
    new Department(){ ID = 4, DeptName = "Admin"}
};

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Left Outer Join باستخدام Query Syntax:

var leftOuterJoin = from dept in departments
                    join emp in employees
                    on dept.ID equals emp.DepartmentID into empGroup
                    from emp in empGroup.DefaultIfEmpty()
                    select new
                    {
                        Department = dept.DeptName,
                        Employee = emp != null ? emp.Name : "No Employee"
                    };

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Left Outer Join باستخدام Method Syntax:

var leftOuterJoin = departments
    .GroupJoin(employees,
               dept => dept.ID,
               emp => emp.DepartmentID,
               (dept, empGroup) => new { dept, empGroup })
    .SelectMany(
        x => x.empGroup.DefaultIfEmpty(),
        (x, emp) => new
        {
            Department = x.dept.DeptName,
            Employee = emp != null ? emp.Name : "No Employee"
        });

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الناتج:

Department: IT, Employee: John  
Department: IT, Employee: Moin  
Department: HR, Employee: Bill  
Department: HR, Employee: Ram  
Department: Finance, Employee: Ron  
Department: Admin, Employee: No Employee

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ملاحظات:

• GroupJoin + DefaultIfEmpty() هما الأساس لتنفيذ Left Join في LINQ.
• تقدر تغير "No Employee" لأي قيمة افتراضية تناسب مشروعك.
• النتيجة دايمًا بترجع كل عناصر القسم، حتى لو مفيش موظفين فيه.