C# Language LINQ Queries Joining multiple sequences
Example
Consider entities Customer, Purchase and PurchaseItem as follows:
public class Customer
{
public string Id { get; set } // A unique Id that identifies customer
public string Name {get; set; }
}
public class Purchase
{
public string Id { get; set }
public string CustomerId {get; set; }
public string Description { get; set; }
}
public class PurchaseItem
{
public string Id { get; set }
public string PurchaseId {get; set; }
public string Detail { get; set; }
}
Consider following sample data for above entities:
var customers = new List<Customer>()
{
new Customer() {
Id = Guid.NewGuid().ToString(),
Name = "Customer1"
},
new Customer() {
Id = Guid.NewGuid().ToString(),
Name = "Customer2"
}
};
var purchases = new List<Purchase>()
{
new Purchase() {
Id = Guid.NewGuid().ToString(),
CustomerId = customers[0].Id,
Description = "Customer1-Purchase1"
},
new Purchase() {
Id = Guid.NewGuid().ToString(),
CustomerId = customers[0].Id,
Description = "Customer1-Purchase2"
},
new Purchase() {
Id = Guid.NewGuid().ToString(),
CustomerId = customers[1].Id,
Description = "Customer2-Purchase1"
},
new Purchase() {
Id = Guid.NewGuid().ToString(),
CustomerId = customers[1].Id,
Description = "Customer2-Purchase2"
}
};
var purchaseItems = new List<PurchaseItem>()
{
new PurchaseItem() {
Id = Guid.NewGuid().ToString(),
PurchaseId= purchases[0].Id,
Detail = "Purchase1-PurchaseItem1"
},
new PurchaseItem() {
Id = Guid.NewGuid().ToString(),
PurchaseId= purchases[1].Id,
Detail = "Purchase2-PurchaseItem1"
},
new PurchaseItem() {
Id = Guid.NewGuid().ToString(),
PurchaseId= purchases[1].Id,
Detail = "Purchase2-PurchaseItem2"
},
new PurchaseItem() {
Id = Guid.NewGuid().ToString(),
PurchaseId= purchases[3].Id,
Detail = "Purchase3-PurchaseItem1"
}
};
Now, consider below linq query:
var result = from c in customers
join p in purchases on c.Id equals p.CustomerId // first join
join pi in purchaseItems on p.Id equals pi.PurchaseId // second join
select new
{
c.Name, p.Description, pi.Detail
};
To output the result of above query:
foreach(var resultItem in result)
{
Console.WriteLine($"{resultItem.Name}, {resultItem.Description}, {resultItem.Detail}");
}
The output of the query would be:
Customer1, Customer1-Purchase1, Purchase1-PurchaseItem1
Customer1, Customer1-Purchase2, Purchase2-PurchaseItem1
Customer1, Customer1-Purchase2, Purchase2-PurchaseItem2
Customer2, Customer2-Purchase2, Purchase3-PurchaseItem1
