Ejemplos de sintaxis de expresiones de consultas: proyección (LINQ to Entities)

Los ejemplos de este tema muestran cómo se utilizan el método Select y las palabras clave From … From … para consultar el modelo AdventureWorks Sales con la sintaxis de las expresiones de consulta. From … From … es el equivalente basado en las consultas del método SelectMany. El modelo AdventureWorks Sales que se usa en estos ejemplos se crea a partir de las tablas Contact, Address, Product, SalesOrderHeader y SalesOrderDetail en la base de datos de ejemplo AdventureWorks.

Los ejemplos de este tema usan las siguientes instrucciones using/Imports:

Option Explicit On
Option Strict On
Imports L2EExamplesVB.AdventureWorksModel
Imports System.Data.Objects
Imports System.Globalization

using System;
using System.Data;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Objects;
using AdventureWorksModel;
using System.Globalization;

Para obtener más información, vea Cómo crear un proyecto de LINQ to Entities en Visual Studio.

Select

Ejemplo

En el ejemplo siguiente se utiliza el método Select para devolver todas las filas de la tabla Product y mostrar los nombres de producto.

Using AWEntities As New AdventureWorksEntities
    Dim products As ObjectQuery(Of Product) = AWEntities.Product

    Dim productsQuery = _
        From product In products _
        Select product

    Console.WriteLine("Product Names:")
    For Each product In productsQuery
        Console.WriteLine(product.Name)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Product> products = AWEntities.Product;
    IQueryable<Product> productsQuery = from product in products
                                        select product;

    Console.WriteLine("Product Names:");
    foreach (var prod in productsQuery)
    {
        Console.WriteLine(prod.Name);
    }
}

Ejemplo

El ejemplo siguiente utiliza Select para devolver una secuencia de nombres de producto solamente.

Using AWEntities As New AdventureWorksEntities
    Dim products As ObjectQuery(Of Product) = AWEntities.Product

    Dim productNames = _
        From p In products _
        Select p.Name

    Console.WriteLine("Product Names:")
    For Each productName In productNames
        Console.WriteLine(productName)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Product> products = AWEntities.Product;

    IQueryable<string> productNames =
        from p in products
        select p.Name;

    Console.WriteLine("Product Names:");
    foreach (String productName in productNames)
    {
        Console.WriteLine(productName);
    }
}

Ejemplo

En el ejemplo siguiente se utiliza el método Select para proyectar las propiedades Product.Name y Product.ProductID en una secuencia de tipos anónimos.

Using AWEntities As New AdventureWorksEntities
    Dim products As ObjectQuery(Of Product) = AWEntities.Product

    Dim query = _
        From product In products _
        Select New With _
        { _
            .ProductId = product.ProductID, _
            .ProductName = product.Name _
        }

    Console.WriteLine("Product Info:")
    For Each productInfo In query
        Console.WriteLine("Product Id: {0} Product name: {1} ", _
                productInfo.ProductId, productInfo.ProductName)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Product> products = AWEntities.Product;
    var query =
        from product in products
        select new
        {
            ProductId = product.ProductID,
            ProductName = product.Name
        };

    Console.WriteLine("Product Info:");
    foreach (var productInfo in query)
    {
        Console.WriteLine("Product Id: {0} Product name: {1} ",
            productInfo.ProductId, productInfo.ProductName);
    }
}

From … From … (SelectMany)

Ejemplo

En el ejemplo siguiente se utiliza From … From … (el equivalente del método SelectMany) para seleccionar todos los pedidos en los que TotalDue es inferior a 500,00.

Using AWEntities As New AdventureWorksEntities
    Dim contacts As ObjectQuery(Of Contact) = AWEntities.Contact
    Dim orders As ObjectQuery(Of SalesOrderHeader) = AWEntities.SalesOrderHeader

    Dim query = _
        From contact In contacts _
        From order In orders _
        Where contact.ContactID = order.Contact.ContactID _
                And order.TotalDue < 500D _
        Select New With _
        { _
            .ContactID = contact.ContactID, _
            .LastName = contact.LastName, _
            .FirstName = contact.FirstName, _
            .OrderID = order.SalesOrderID, _
            .Total = order.TotalDue _
        }

    For Each smallOrder In query
        Console.WriteLine("Contact ID: {0} Name: {1}, {2} Order ID: {3} Total Due: ${4} ", _
            smallOrder.ContactID, smallOrder.LastName, smallOrder.FirstName, _
            smallOrder.OrderID, smallOrder.Total)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Contact> contacts = AWEntities.Contact;
    ObjectQuery<SalesOrderHeader> orders = AWEntities.SalesOrderHeader;

    var query =
        from contact in contacts
        from order in orders
        where contact.ContactID == order.Contact.ContactID
            && order.TotalDue < 500.00M
        select new
        {
            ContactID = contact.ContactID,
            LastName = contact.LastName,
            FirstName = contact.FirstName,
            OrderID = order.SalesOrderID,
            Total = order.TotalDue
        };

    foreach (var smallOrder in query)
    {
        Console.WriteLine("Contact ID: {0} Name: {1}, {2} Order ID: {3} Total Due: ${4} ",
            smallOrder.ContactID, smallOrder.LastName, smallOrder.FirstName,
            smallOrder.OrderID, smallOrder.Total);
    }
}

Ejemplo

En el ejemplo siguiente se utiliza From … From … (el equivalente del método SelectMany) para seleccionar todos los pedidos efectuados a partir del 1 de octubre de 2002.

Using AWEntities As New AdventureWorksEntities
    Dim contacts As ObjectQuery(Of Contact) = AWEntities.Contact
    Dim orders As ObjectQuery(Of SalesOrderHeader) = AWEntities.SalesOrderHeader

    Dim query = _
        From contact In contacts _
        From order In orders _
        Where contact.ContactID = order.Contact.ContactID _
                And order.OrderDate >= New DateTime(2002, 10, 1) _
        Select New With _
        { _
            .ContactID = contact.ContactID, _
            .LastName = contact.LastName, _
            .FirstName = contact.FirstName, _
            .OrderID = order.SalesOrderID, _
            .OrderDate = order.OrderDate _
        }

    For Each order In query
        Console.WriteLine("Contact ID: {0} Name: {1}, {2} Order ID: {3} Order date: {4:d} ", _
            order.ContactID, order.LastName, order.FirstName, _
            order.OrderID, order.OrderDate)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Contact> contacts = AWEntities.Contact;
    ObjectQuery<SalesOrderHeader> orders = AWEntities.SalesOrderHeader;

    var query =
        from contact in contacts
        from order in orders
        where contact.ContactID == order.Contact.ContactID
            && order.OrderDate >= new DateTime(2002, 10, 1)
        select new
        {
            ContactID = contact.ContactID,
            LastName = contact.LastName,
            FirstName = contact.FirstName,
            OrderID = order.SalesOrderID,
            OrderDate = order.OrderDate
        };

    foreach (var order in query)
    {
        Console.WriteLine("Contact ID: {0} Name: {1}, {2} Order ID: {3} Order date: {4:d} ",
            order.ContactID, order.LastName, order.FirstName,
            order.OrderID, order.OrderDate);
    }
}

Ejemplo

En el ejemplo siguiente se utiliza From … From … (el equivalente al método SelectMany) para seleccionar todos los pedidos en los que el total del pedido es superior a 10000,00 y se utiliza la asignación From para evitar que se solicite dos veces el total.

Using AWEntities As New AdventureWorksEntities
    Dim contacts As ObjectQuery(Of Contact) = AWEntities.Contact
    Dim orders As ObjectQuery(Of SalesOrderHeader) = AWEntities.SalesOrderHeader

    Dim query = _
        From contact In contacts _
        From order In orders _
        Let total = order.TotalDue _
        Where contact.ContactID = order.Contact.ContactID _
                And total >= 10000D _
        Select New With _
        { _
            .ContactID = contact.ContactID, _
            .LastName = contact.LastName, _
            .OrderID = order.SalesOrderID, _
            total _
        }

    For Each order In query
        Console.WriteLine("Contact ID: {0} Last name: {1} Order ID: {2} Total: {3}", _
                order.ContactID, order.LastName, order.OrderID, order.total)
    Next
End Using

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectQuery<Contact> contacts = AWEntities.Contact;
    ObjectQuery<SalesOrderHeader> orders = AWEntities.SalesOrderHeader;

    var query =
        from contact in contacts
        from order in orders
        let total = order.TotalDue
        where contact.ContactID == order.Contact.ContactID
            && total >= 10000.0M
        select new
        {
            ContactID = contact.ContactID,
            LastName = contact.LastName,
            OrderID = order.SalesOrderID,
            total
        };

    foreach (var order in query)
    {
        Console.WriteLine("Contact ID: {0} Last name: {1} Order ID: {2} Total: {3}",
            order.ContactID, order.LastName, order.OrderID, order.total);
    }
}

Vea también

Conceptos

Ejemplos de sintaxis de expresión de consultas (LINQ to Entities)