linq Standard Query Operators Join Operations

Example

A join of two data sources is the association of objects in one data source with objects that share a common attribute in another data source.

Join

Joins two sequences based on key selector functions and extracts pairs of values.

Method Syntax

// Join

class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

class Order
{
    public string Description { get; set; }
    public int CustomerId { get; set; }
}
...

var customers = new Customer[] 
{
    new Customer { Id = 1, Name = "C1" },
    new Customer { Id = 2, Name = "C2" },
    new Customer { Id = 3, Name = "C3" }
};

var orders = new Order[]
{
    new Order { Description = "O1", CustomerId = 1 },
    new Order { Description = "O2", CustomerId = 1 },
    new Order { Description = "O3", CustomerId = 2 },
    new Order { Description = "O4", CustomerId = 3 },
};

var join = customers.Join(orders, c => c.Id, o => o.CustomerId, (c, o) => c.Name + "-" + o.Description);

// join = { "C1-O1", "C1-O2", "C2-O3", "C3-O4" }

Query Syntax

// join … in … on … equals …

var join = from c in customers
           join o in orders
           on c.Id equals o.CustomerId
           select o.Description + "-" + c.Name;

// join = { "O1-C1", "O2-C1", "O3-C2", "O4-C3" }

---

GroupJoin

Joins two sequences based on key selector functions and groups the resulting matches for each element.

Method Syntax

// GroupJoin

var groupJoin = customers.GroupJoin(orders,
                                    c => c.Id, 
                                    o => o.CustomerId, 
                                    (c, ors) => c.Name + "-" + string.Join(",", ors.Select(o => o.Description)));

// groupJoin = { "C1-O1,O2", "C2-O3", "C3-O4" }

Query Syntax

// join … in … on … equals … into …

var groupJoin = from c in customers
                join o in orders               
                on c.Id equals o.CustomerId
                into customerOrders
                select string.Join(",", customerOrders.Select(o => o.Description)) + "-" + c.Name;

// groupJoin = { "O1,O2-C1", "O3-C2", "O4-C3" }

Zip

Applies a specified function to the corresponding elements of two sequences, producing a sequence of the results.

var numbers = new [] { 1, 2, 3, 4, 5, 6 };
var words = new [] { "one", "two", "three" };

var numbersWithWords =
    numbers
    .Zip(
        words,
        (number, word) => new { number, word });

// Results

//| number | word   |
//| ------ | ------ |
//| 1      |  one   |
//| 2      |  two   |
//| 3      |  three |