How to: Formulate Projections (LINQ to SQL)
The following examples show how the select statement in C# and Select statement in Visual Basic can be combined with other features to form query projections.
Example
The following example uses the Select clause in Visual Basic (select clause in C#) to return a sequence of contact names for Customers.
Dim nameQuery = From cust In db.Customers _
Select cust.ContactName
var nameQuery =
from cust in db.Customers
select cust.ContactName;
The following example uses the Select clause in Visual Basic (select clause in C#) and anonymous types to return a sequence of contact names and telephone numbers for Customers.
Dim infoQuery = From cust In db.Customers _
Select cust.ContactName, cust.Phone
var infoQuery =
from cust in db.Customers
select new { cust.ContactName, cust.Phone };
The following example uses the Select clause in Visual Basic (select clause in C#) and anonymous types to return a sequence of names and telephone numbers for employees. The FirstName and LastName fields are combined into a single field (Name), and the HomePhone field is renamed to Phone in the resulting sequence.
Dim info2Query = From emp In db.Employees _
Select Name = emp.FirstName & " " & emp.LastName, _
Phone = emp.HomePhone
var info2Query =
from emp in db.Employees
select new
{
Name = emp.FirstName + " " + emp.LastName,
Phone = emp.HomePhone
};
The following example uses the Select clause in Visual Basic (select clause in C#) and anonymous types to return a sequence of all ProductIDs and a calculated value named HalfPrice. This value is set to the UnitPrice divided by 2.
Dim specialQuery = From prod In db.Products _
Select prod.ProductID, HalfPrice = CDec(prod.UnitPrice) / 2
var specialQuery =
from prod in db.Products
select new { prod.ProductID, HalfPrice = prod.UnitPrice / 2 };
The following example uses the Select clause in Visual Basic (select clause in C#) and a conditional statement to return a sequence of product name and product availability.
Dim prodQuery = From prod In db.Products _
Select prod.ProductName, Availability = _
If(prod.UnitsInStock - prod.UnitsOnOrder < 0, _
"Out Of Stock", "In Stock")
var prodQuery =
from prod in db.Products
select new
{
prod.ProductName,
Availability =
prod.UnitsInStock - prod.UnitsOnOrder < 0
? "Out Of Stock" : "In Stock"
};
The following example uses a Visual Basic Select clause (select clause in C#) and a known type (Name) to return a sequence of the names of employees.
Public Class Name
Public FirstName As String
Public LastName As String
End Class
Dim db As New Northwnd("c:\northwnd.mdf")
Dim empQuery = From emp In db.Employees _
Select New Name With {.FirstName = emp.FirstName, .LastName = _
emp.LastName}
public class Name
{
public string FirstName = "";
public string LastName = "";
}
void empMethod()
{
Northwnd db = new Northwnd(@"c:\northwnd.mdf");
var empQuery =
from emp in db.Employees
select new Name
{
FirstName = emp.FirstName,
LastName = emp.LastName
};
}
The following example uses Select and Where in Visual Basic (select and where in C#) to return a filtered sequence of contact names for customers in London.
Dim contactQuery = _
From cust In db.Customers _
Where cust.City = "London" _
Select cust.ContactName
var contactQuery =
from cust in db.Customers
where cust.City == "London"
select cust.ContactName;
The following example uses a Select clause in Visual Basic (select clause in C#) and anonymous types to return a shaped subset of the data about customers.
Dim custQuery = From cust In db.Customers _
Select cust.CustomerID, CompanyInfo = New With {cust.CompanyName, _
cust.City, cust.Country}, ContactInfo = _
New With {cust.ContactName, cust.ContactTitle}
var custQuery =
from cust in db.Customers
select new
{
cust.CustomerID,
CompanyInfo = new { cust.CompanyName, cust.City, cust.Country },
ContactInfo = new { cust.ContactName, cust.ContactTitle }
};
The following example uses nested queries to return the following results:
A sequence of all orders and their corresponding OrderIDs.
A subsequence of the items in the order for which there is a discount.
The amount of money saved if the cost of shipping is not included.
Dim ordQuery = From ord In db.Orders _
Select ord.OrderID, DiscountedProducts = _
(From od In ord.OrderDetails _
Where od.Discount > 0.0 _
Select od), _
FreeShippingDiscount = ord.Freight
var ordQuery =
from ord in db.Orders
select new
{
ord.OrderID,
DiscountedProducts =
from od in ord.OrderDetails
where od.Discount > 0.0
select od,
FreeShippingDiscount = ord.Freight
};