How to: Formulate Joins and Cross-Product Queries (LINQ to SQL)
The following examples show how to combine results from multiple tables.
Example
The following example uses foreign key navigation in the From clause in Visual Basic (from clause in C#) to select all orders for customers in London.
Dim infoQuery = _
From cust In db.Customers, ord In cust.Orders _
Where cust.City = "London" _
Select ord
var infoQuery =
from cust in db.Customers
from ord in cust.Orders
where cust.City == "London"
select ord;
The following example uses foreign key navigation in the Where clause in Visual Basic (where clause in C#) to filter for out-of-stock Products whose Supplier is in the United States.
Dim infoQuery = _
From prod In db.Products _
Where prod.Supplier.Country = "USA" AndAlso _
CShort(prod.UnitsInStock) = 0 _
Select prod
var infoQuery =
from prod in db.Products
where prod.Supplier.Country == "USA" && prod.UnitsInStock == 0
select prod;
The following example uses foreign key navigation in the From clause in Visual Basic (from clause in C#) to filter for employees in Seattle and to list their territories.
var infoQuery =
from emp in db.Employees
from empterr in emp.EmployeeTerritories
where emp.City == "Seattle"
select new
{
emp.FirstName,
emp.LastName,
empterr.Territory.TerritoryDescription
};
The following example uses foreign key navigation in the Select clause in Visual Basic (select clause in C#) to filter for pairs of employees where one employee reports to the other and where both employees are from the same City.
Dim infoQuery = _
From e1 In db.Employees, e2 In e1.Employees _
Where e1.City = e2.City _
Select FirstName1 = e1.FirstName, _
LastName1 = e1.LastName, FirstName2 = e2.FirstName, _
LastName2 = e2.LastName, e1.City
var infoQuery =
from emp1 in db.Employees
from emp2 in emp1.Employees
where emp1.City == emp2.City
select new
{
FirstName1 = emp1.FirstName,
LastName1 = emp1.LastName,
FirstName2 = emp2.FirstName,
LastName2 = emp2.LastName,
emp1.City
};
The following Visual Basic example looks for all customers and orders, makes sure that the orders are matched to customers, and guarantees that for every customer in that list, a contact name is provided.
Dim q1 = From c In db.Customers, o In db.Orders _
Where c.CustomerID = o.CustomerID _
Select c.CompanyName, o.ShipRegion
' Note that because the O/R designer generates class
' hierarchies for database relationships for you,
' the following code has the same effect as the above
' and is shorter:
Dim q2 = From c In db.Customers, o In c.Orders _
Select c.CompanyName, o.ShipRegion
For Each nextItem In q2
Console.WriteLine("{0} {1}", nextItem.CompanyName, _
nextItem.ShipRegion)
Next
The following example explicitly joins two tables and projects results from both tables.
Dim q = From c In db.Customers _
Group Join o In db.Orders On c.CustomerID Equals o.CustomerID _
Into orders = Group _
Select c.ContactName, OrderCount = orders.Count()
var q =
from c in db.Customers
join o in db.Orders on c.CustomerID equals o.CustomerID
into orders
select new { c.ContactName, OrderCount = orders.Count() };
The following example explicitly joins three tables and projects results from each of them.
Dim q = From c In db.Customers _
Group Join o In db.Orders On c.CustomerID Equals o.CustomerID _
Into ords = Group _
Group Join e In db.Employees On c.City Equals e.City _
Into emps = Group _
Select c.ContactName, ords = ords.Count(), emps = emps.Count()
var q =
from c in db.Customers
join o in db.Orders on c.CustomerID equals o.CustomerID
into ords
join e in db.Employees on c.City equals e.City into emps
select new
{
c.ContactName,
ords = ords.Count(),
emps = emps.Count()
};
The following example shows how to achieve a LEFT OUTER JOIN by using DefaultIfEmpty(). The DefaultIfEmpty() method returns null when there is no Order for the Employee.
Dim q = From e In db.Employees() _
Group Join o In db.Orders On e Equals o.Employee Into ords _
= Group _
From o In ords.DefaultIfEmpty() _
Select e.FirstName, e.LastName, Order = o
var q =
from e in db.Employees
join o in db.Orders on e equals o.Employee into ords
from o in ords.DefaultIfEmpty()
select new { e.FirstName, e.LastName, Order = o };
The following example projects a let expression resulting from a join.
Dim q = From c In db.Customers _
Group Join o In db.Orders On c.CustomerID Equals o.CustomerID _
Into ords = Group _
Let z = c.City + c.Country _
From o In ords _
Select c.ContactName, o.OrderID, z
var q =
from c in db.Customers
join o in db.Orders on c.CustomerID equals o.CustomerID
into ords
let z = c.City + c.Country
from o in ords
select new { c.ContactName, o.OrderID, z };
The following example shows a join with a composite key.
Dim q = From o In db.Orders _
From p In db.Products _
Group Join d In db.OrderDetails On New With {o.OrderID, _
p.ProductID} _
Equals New With {d.OrderID, d.ProductID} Into details _
= Group _
From d In details _
Select o.OrderID, p.ProductID, d.UnitPrice
var q =
from o in db.Orders
from p in db.Products
join d in db.OrderDetails
on new { o.OrderID, p.ProductID } equals new
{
d.OrderID,
d.ProductID
} into details
from d in details
select new { o.OrderID, p.ProductID, d.UnitPrice };
The following example shows how to construct a join where one side is nullable and the other is not.
Dim q = From o In db.Orders _
Group Join e In db.Employees On o.EmployeeID _
Equals e.EmployeeID Into emps = Group _
From e In emps _
Select o.OrderID, e.FirstName
var q =
from o in db.Orders
join e in db.Employees
on o.EmployeeID equals (int?)e.EmployeeID into emps
from e in emps
select new { o.OrderID, e.FirstName };