基于方法的查询语法(1)

实体框架(Entity Framework )是 ADO.NET 中的一套支持开发面向数据的软件应用程序的技术。

 LINQ to Entities 提供语言集成查询 (LINQ) 支持，它允许开发人员使用 Visual Basic 或 Visual C# 根据实体框架概念模型编写查询。针对实体框架的查询由针对对象上下文执行的命令目录树查询表示。LINQ to Entities 将语言集成查询 (LINQ) 查询转换为命令目录树查询，针对实体框架执行这些查询，并返回可同时由实体框架和 LINQ 使用的对象。

下面列出一些基于方法的查询语法示例(C#)：

投影  Select | SelectMany

筛选 Where | Where…Contains

排序 ThenBy | ThenByDescending

聚合运算符 Average | Count | LongCount | Max | Min | Sum

分区 Skip | Take

转换 ToArray | ToDictionary | ToList

联接运算符 GroupJoin | Join

元素运算符 First

分组 GroupBy

导航关系

Select

以下示例使用 Select 方法以将 Product.Name 和 Product.ProductID 属性投影到一系列匿名类型。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    var query = AWEntities.Products
        .Select(product => 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);
    }
}

以下示例使用 Select 方法以只返回一系列产品名称。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    IQueryable productNames = AWEntities.Products
        .Select(p => p.Name);

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

SelectMany

以下示例使用 SelectMany 方法以选择 TotalDue 低于 500.00 的所有订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet contacts = AWEntities.Contacts;
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
    contacts.SelectMany(
        contact => orders.Where(order =>
            (contact.ContactID == order.Contact.ContactID)
                && order.TotalDue < 500.00M)
            .Select(order => 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);
    }
}

以下示例使用 SelectMany 方法以选择在 2002 年 10 月 1 或此日期之后发出的所有订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet contacts = AWEntities.Contacts;
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
    contacts.SelectMany(
        contact => orders.Where(order =>
            (contact.ContactID == order.Contact.ContactID)
                && order.OrderDate >= new DateTime(2002, 10, 1))
            .Select(order => 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);
    }
}

Where

以下示例返回所有联机订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    var onlineOrders = AWEntities.SalesOrderHeaders
        .Where(order => order.OnlineOrderFlag == true)
        .Select(s => new { s.SalesOrderID, s.OrderDate, s.SalesOrderNumber });

    foreach (var onlineOrder in onlineOrders)
    {
        Console.WriteLine("Order ID: {0} Order date: {1:d} Order number: {2}",
            onlineOrder.SalesOrderID,
            onlineOrder.OrderDate,
            onlineOrder.SalesOrderNumber);
    }
}

以下示例返回订单数量大于 2 且小于 6 的订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    var query = AWEntities.SalesOrderDetails
        .Where(order => order.OrderQty > 2 && order.OrderQty < 6)
        .Select(s => new { s.SalesOrderID, s.OrderQty });

    foreach (var order in query)
    {
        Console.WriteLine("Order ID: {0} Order quantity: {1}",
            order.SalesOrderID, order.OrderQty);
    }
}

以下示例返回所有红色产品。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    var query = AWEntities.Products
        .Where(product => product.Color == "Red")
        .Select(p => new { p.Name, p.ProductNumber, p.ListPrice });

    foreach (var product in query)
    {
        Console.WriteLine("Name: {0}", product.Name);
        Console.WriteLine("Product number: {0}", product.ProductNumber);
        Console.WriteLine("List price: ${0}", product.ListPrice);
        Console.WriteLine("");
    }
}

Where…Contains

以下示例将一个数组用作 Where¡Contains 子句的一部分，以查找 ProductModelID 与数组中的值匹配的所有产品。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    int?[] productModelIds = { 19, 26, 118 };
    var products = AWEntities.Products.
        Where(p => productModelIds.Contains(p.ProductModelID));

    foreach (var product in products)
    {
        Console.WriteLine("{0}: {1}", product.ProductModelID, product.ProductID);
    }
}

作为 Where¡Contains 子句中谓词的一部分，您可以使用 Array、List<(Of <(<'T>)>)> 或实现IEnumerable<(Of <(<'T>)>)> 接口的任何类型的集合。 还可以在 LINQ to Entities 查询中声明和初始化集合。

以下示例声明并初始化 Where¡Contains 子句中的数组，以查找 ProductModelID 或 Size 与数组中的值匹配的所有产品。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    var products = AWEntities.Products.
        Where(p => (new int?[] { 19, 26, 18 }).Contains(p.ProductModelID) ||
                   (new string[] { "L", "XL" }).Contains(p.Size));

    foreach (var product in products)
    {
        Console.WriteLine("{0}: {1}, {2}", product.ProductID,
                                           product.ProductModelID,
                                           product.Size);
    }
}

ThenBy

采用基于方法的查询语法的以下示例使用 OrderBy 和 ThenBy 以返回先按姓氏后按名字排序的联系人列表。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    IQueryable sortedContacts = AWEntities.Contacts
        .OrderBy(c => c.LastName)
        .ThenBy(c => c.FirstName)
        .Select(c => c);

    Console.WriteLine("The list of contacts sorted by last name then by first name:");
    foreach (Contact sortedContact in sortedContacts)
    {
        Console.WriteLine(sortedContact.LastName + ", " + sortedContact.FirstName);
    }
}

ThenByDescending

以下示例使用 OrderBy 和 ThenByDescending 方法以首先按标价排序，然后执行产品名称的降序排序。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    IOrderedQueryable query = products
        .OrderBy(product => product.ListPrice)
        .ThenByDescending(product => product.Name);

    foreach (Product product in query)
    {
        Console.WriteLine("Product ID: {0} Product Name: {1} List Price {2}",
            product.ProductID,
            product.Name,
            product.ListPrice);
    }
}

Average

以下示例使用 Average 方法来查找产品的平均标价。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    Decimal averageListPrice =
        products.Average(product => product.ListPrice);

    Console.WriteLine("The average list price of all the products is ${0}",
        averageListPrice);
}

以下示例使用 Average 方法以查找每种样式的产品的平均标价。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    var query = from product in products
                group product by product.Style into g
                select new
                {
                    Style = g.Key,
                    AverageListPrice =
                        g.Average(product => product.ListPrice)
                };

    foreach (var product in query)
    {
        Console.WriteLine("Product style: {0} Average list price: {1}",
            product.Style, product.AverageListPrice);
    }
}

以下示例使用 Average 方法以查找平均应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    Decimal averageTotalDue = orders.Average(order => order.TotalDue);
    Console.WriteLine("The average TotalDue is {0}.", averageTotalDue);
}

以下示例使用 Average 方法以获取每个联系人 ID 的平均应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        select new
        {
            Category = g.Key,
            averageTotalDue = g.Average(order => order.TotalDue)
        };

    foreach (var order in query)
    {
        Console.WriteLine("ContactID = {0} \t Average TotalDue = {1}",
            order.Category, order.averageTotalDue);
    }
}

以下示例使用 Average 方法以针对每个联系人获取具有平均应付款总计的订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        let averageTotalDue = g.Average(order => order.TotalDue)
        select new
        {
            Category = g.Key,
            CheapestProducts =
                g.Where(order => order.TotalDue == averageTotalDue)
        };

    foreach (var orderGroup in query)
    {
        Console.WriteLine("ContactID: {0}", orderGroup.Category);
        foreach (var order in orderGroup.CheapestProducts)
        {
            Console.WriteLine("Average total due for SalesOrderID {1} is: {0}",
                order.TotalDue, order.SalesOrderID);
        }
        Console.Write("\n");
    }
}

Count

以下示例使用 Count 方法以返回 Product 表中的产品数量。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    int numProducts = products.Count();

    Console.WriteLine("There are {0} products.", numProducts);
}

以下示例使用 Count 方法以返回联系人 ID 的列表和每个联系人 ID 所具有的订单数。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet contacts = AWEntities.Contacts;

    //Can't find field SalesOrderContact
    var query =
        from contact in contacts
        select new
        {
            CustomerID = contact.ContactID,
            OrderCount = contact.SalesOrderHeaders.Count()
        };

    foreach (var contact in query)
    {
        Console.WriteLine("CustomerID = {0} \t OrderCount = {1}",
            contact.CustomerID,
            contact.OrderCount);
    }
}

以下示例按颜色对产品进行分组，并使用 Count 方法以返回每个颜色组中的产品数量。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    var query =
        from product in products
        group product by product.Color into g
        select new { Color = g.Key, ProductCount = g.Count() };

    foreach (var product in query)
    {
        Console.WriteLine("Color = {0} \t ProductCount = {1}",
            product.Color,
            product.ProductCount);
    }
}

LongCount

以下示例以长整型获取联系人计数。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet contacts = AWEntities.Contacts;

    long numberOfContacts = contacts.LongCount();
    Console.WriteLine("There are {0} Contacts", numberOfContacts);
}

Max

以下示例使用 Max 方法以获取最大应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    Decimal maxTotalDue = orders.Max(w => w.TotalDue);
    Console.WriteLine("The maximum TotalDue is {0}.",
        maxTotalDue);
}

以下示例使用 Max 方法以获取每个联系人 ID 的最大应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        select new
        {
            Category = g.Key,
            maxTotalDue =
                g.Max(order => order.TotalDue)
        };

    foreach (var order in query)
    {
        Console.WriteLine("ContactID = {0} \t Maximum TotalDue = {1}",
            order.Category, order.maxTotalDue);
    }
}

以下示例使用 Max 方法以针对每个联系人 ID 获取具有最大应付款总计的订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        let maxTotalDue = g.Max(order => order.TotalDue)
        select new
        {
            Category = g.Key,
            CheapestProducts =
                g.Where(order => order.TotalDue == maxTotalDue)
        };

    foreach (var orderGroup in query)
    {
        Console.WriteLine("ContactID: {0}", orderGroup.Category);
        foreach (var order in orderGroup.CheapestProducts)
        {
            Console.WriteLine("MaxTotalDue {0} for SalesOrderID {1}: ",
                order.TotalDue,
                order.SalesOrderID);
        }
        Console.Write("\n");
    }
}

Min

以下示例使用 Min 方法以获取最小应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    Decimal smallestTotalDue = orders.Min(totalDue => totalDue.TotalDue);
    Console.WriteLine("The smallest TotalDue is {0}.",
        smallestTotalDue);
}

以下示例使用 Min 方法以获取每个联系人 ID 的最小应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        select new
        {
            Category = g.Key,
            smallestTotalDue =
                g.Min(order => order.TotalDue)
        };

    foreach (var order in query)
    {
        Console.WriteLine("ContactID = {0} \t Minimum TotalDue = {1}",
            order.Category, order.smallestTotalDue);
    }
}

以下示例使用 Min 方法以针对每个联系人获取具有最小应付款总计的订单。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        let minTotalDue = g.Min(order => order.TotalDue)
        select new
        {
            Category = g.Key,
            smallestTotalDue =
                g.Where(order => order.TotalDue == minTotalDue)
        };

    foreach (var orderGroup in query)
    {
        Console.WriteLine("ContactID: {0}", orderGroup.Category);
        foreach (var order in orderGroup.smallestTotalDue)
        {
            Console.WriteLine("Mininum TotalDue {0} for SalesOrderID {1}: ",
                order.TotalDue,
                order.SalesOrderID);
        }
        Console.Write("\n");
    }
}

Sum

以下示例使用 Sum 方法以获取 SalesOrderDetail 表中订单数量的总数。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderDetails;

    double totalOrderQty = orders.Sum(o => o.OrderQty);
    Console.WriteLine("There are a total of {0} OrderQty.",
        totalOrderQty);
}

以下示例使用 Sum 方法以获取每个联系人 ID 的应付款总计。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet orders = AWEntities.SalesOrderHeaders;

    var query =
        from order in orders
        group order by order.Contact.ContactID into g
        select new
        {
            Category = g.Key,
            TotalDue = g.Sum(order => order.TotalDue)
        };

    foreach (var order in query)
    {
        Console.WriteLine("ContactID = {0} \t TotalDue sum = {1}",
            order.Category, order.TotalDue);
    }
}

Skip

以下示例使用 Skip<(Of <<'(TSource>)>>) 方法以获取 Contact 表中除前三个联系人之外的所有联系人。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    // LINQ to Entities only supports Skip on ordered collections.
    IOrderedQueryable products = AWEntities.Products
            .OrderBy(p => p.ListPrice);

    IQueryable allButFirst3Products = products.Skip(3);

    Console.WriteLine("All but first 3 products:");
    foreach (Product product in allButFirst3Products)
    {
        Console.WriteLine("Name: {0} \t ID: {1}",
            product.Name,
            product.ProductID);
    }
}

以下示例使用 Skip<(Of <<'(TSource>)>>) 方法以获取 Seattle 的前两个地址之外的所有地址。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet

addresses = AWEntities.Addresses;

ObjectSet orders = AWEntities.SalesOrderHeaders;

//LINQ to Entities only supports Skip on ordered collections.

var query = (

from address in addresses

from order in orders

where address.AddressID == order.Address.AddressID

&& address.City == "Seattle"

orderby order.SalesOrderID

select new {

City = address.City, OrderID = order.SalesOrderID, OrderDate = order.OrderDate

}).Skip(2);

Console.WriteLine("All but first 2 orders in Seattle:");

foreach (var order in query)

{

Console.WriteLine("City: {0} Order ID: {1} Total Due: {2:d}",

order.City, order.OrderID, order.OrderDate);

} }

Take

以下示例使用 Take<(Of <<'(TSource>)>>) 方法以只从 Contact 表中获取前五个联系人。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet contacts = AWEntities.Contacts;

    IQueryable first5Contacts = contacts.Take(5);

    Console.WriteLine("First 5 contacts:");
    foreach (Contact contact in first5Contacts)
    {
        Console.WriteLine("Title = {0} \t FirstName = {1} \t Lastname = {2}",
            contact.Title,
            contact.FirstName,
            contact.LastName);
    }
}

以下示例使用 Take<(Of <<'(TSource>)>>) 方法以获取 Seattle 的前三个地址。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet

addresses = AWEntities.Addresses;

ObjectSet orders = AWEntities.SalesOrderHeaders;

var query = (

from address in addresses

from order in orders

where address.AddressID == order.Address.AddressID

&& address.City == "Seattle"

select new {

City = address.City, OrderID = order.SalesOrderID, OrderDate = order.OrderDate

}).Take(3);

Console.WriteLine("First 3 orders in Seattle:");

foreach (var order in query)

{

Console.WriteLine("City: {0} Order ID: {1} Total Due: {2:d}",

order.City, order.OrderID, order.OrderDate);

} }

ToArray

下面的示例使用 ToArray<(Of <<'(TSource>)>>) 方法立即将序列计算为数组。

using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
    ObjectSet products = AWEntities.Products;

    Product[] prodArray = (
        from product in products
        orderby product.ListPrice descending
        select product).ToArray();

    Console.WriteLine("Every price from highest to lowest:");
    foreach (Product product in prodArray)
    {
        Console.WriteLine(product.ListPrice);
    }
}