LINQ Join Query

Here are some common LINQ queries related to joins, demonstrating how to combine data from multiple collections using various join techniques:

1. Inner Join

Combines elements from two collections based on a matching key.

var orders = new List<Order>
{
    new Order { OrderId = 1, CustomerId = 1 },
    new Order { OrderId = 2, CustomerId = 2 }
};

var customers = new List<Customer>
{
    new Customer { CustomerId = 1, Name = "Rohatash" },
    new Customer { CustomerId = 2, Name = "Mohit" }
};

var orderDetails = from order in orders
                    join customer in customers
                    on order.CustomerId equals customer.CustomerId
                    select new
                    {
                        OrderId = order.OrderId,
                        CustomerName = customer.Name
                    };

// Output: { OrderId = 1, CustomerName = "Rohatash" }, { OrderId = 2, CustomerName = "Mohit" }

2. Left Outer Join

Includes all elements from the left collection and matching elements from the right collection. Elements from the left collection without matches in the right collection will still be included.

var employees = new List<Employee>
{
    new Employee { EmployeeId = 1, Name = "Rohatash" },
    new Employee { EmployeeId = 2, Name = "Mohit" }
};

var projects = new List<Project>
{
    new Project { ProjectId = 1, EmployeeId = 1, ProjectName = "Project A" }
};

var employeeProjects = from employee in employees
                       join project in projects
                       on employee.EmployeeId equals project.EmployeeId into employeeProjectGroup
                       from project in employeeProjectGroup.DefaultIfEmpty()
                       select new
                       {
                           EmployeeName = employee.Name,
                           ProjectName = project?.ProjectName ?? "No Project"
                       };

// Output: { EmployeeName = "Rohatash", ProjectName = "Project A" }, { EmployeeName = "Mohit", ProjectName = "No Project" }

3. Group Join

Groups elements from the second collection by a key, and then joins these groups to the elements of the first collection.

var customers = new List<Customer>
{
    new Customer { CustomerId = 1, Name = "Rohatash" },
    new Customer { CustomerId = 2, Name = "Mohit" }
};

var orders = new List<Order>
{
    new Order { OrderId = 1, CustomerId = 1 },
    new Order { OrderId = 2, CustomerId = 1 },
    new Order { OrderId = 3, CustomerId = 2 }
};

var customerOrders = from customer in customers
                     join order in orders
                     on customer.CustomerId equals order.CustomerId into customerOrderGroup
                     select new
                     {
                         CustomerName = customer.Name,
                         Orders = customerOrderGroup.ToList()
                     };

// Output: { CustomerName = "Rohatash", Orders = [{ OrderId = 1 }, { OrderId = 2 }] }, { CustomerName = "Mohit", Orders = [{ OrderId = 3 }] }

4. Cross Join (Cartesian Product)

Combines every element from the first collection with every element from the second collection.

var colors = new List<string> { "Red", "Green", "Blue" };
var sizes = new List<string> { "Small", "Medium", "Large" };

var colorSizeCombinations = from color in colors
                            from size in sizes
                            select new
                            {
                                Color = color,
                                Size = size
                            };

// Output: { Color = "Red", Size = "Small" }, { Color = "Red", Size = "Medium" }, ...