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The author:(作者)归海一刀published in(发表于) 2014/1/30 1:03:22 LINQ体验(7)——LINQ,to,SQL语句之Group,ByHaving和Exists_[Asp.Net教程]
LINQ体验(7)——LINQ to SQL语句之Group By/Having和Exists/In/Any/All/Contains
我们继续讲解LINQ语句,我昨天发现了LINQ in Action 电子书,就共享了LINQ in Action 电子书这本书。大家也可以看看。这篇我们来讨论Group By/Having操作符和Exists/In/Any/All/Contains操作符。
适用场景:分组数据,为我们查找数据缩小范围。
说明:分配并返回对传入参数进行分组操作后的可枚举对象。分组;延迟
var q =from p in db.Productsgroup p by p.CategoryID into gselect g;
说明:from p in db.Products 表示从表中将产品对象取出来。group p by p.CategoryID into g表示对p按CategoryID字段归类。其结果命名为g,一旦重新命名,p的作用域就结束了,所以,最后select时,只能select g。当然,也不必重新命名可以这样写:
var q =from p in db.Productsgroup p by p.CategoryID;
我们用示意图表示:
如果想遍历某类别中所有记录,这样:
foreach (var gp in q){if (gp.Key == 2){foreach (var item in gp){//do something}}}
var q =from p in db.Productsgroup p by p.CategoryID into gselect new { CategoryID = g.Key, g };
说明:在这句LINQ语句中,有2个property:CategoryID和g。这个匿名类,其实质是对返回结果集重新进行了包装。把g的property封装成一个完整的分组。如下图所示:
如果想遍历某匿名类中所有记录,要这么做:
foreach (var gp in q){if (gp.CategoryID == 2){foreach (var item in gp){//do something}}}
说明:先按CategoryID归类,判断各个分类产品中单价最大的Products。取出CategoryID值,并把UnitPrice值赋给MaxPrice。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,MaxPrice = g.Max(p => p.UnitPrice)};
说明:先按CategoryID归类,判断各个分类产品中单价最小的Products。取出CategoryID值,并把UnitPrice值赋给MinPrice。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,MinPrice = g.Min(p => p.UnitPrice)};
说明:先按CategoryID归类,取出CategoryID值和各个分类产品中单价的平均值。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,AveragePrice = g.Average(p => p.UnitPrice)};
说明:先按CategoryID归类,取出CategoryID值和各个分类产品中单价的总和。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,TotalPrice = g.Sum(p => p.UnitPrice)};
说明:先按CategoryID归类,取出CategoryID值和各个分类产品的数量。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,NumProducts = g.Count()};
说明:先按CategoryID归类,取出CategoryID值和各个分类产品的断货数量。 Count函数里,使用了Lambda表达式,Lambda表达式中的p,代表这个组里的一个元素或对象,即某一个产品。
var q =from p in db.Productsgroup p by p.CategoryID into gselect new {g.Key,NumProducts = g.Count(p => p.Discontinued)};
说明:在翻译成SQL语句时,在最外层嵌套了Where条件。
var q =from p in db.Productsgroup p by p.CategoryID into gwhere g.Count() >= 10select new {g.Key,ProductCount = g.Count()};
语句描述:根据产品的―ID分组,查询产品数量大于10的ID和产品数量。
说明:既按产品的分类,又按供应商分类。在by后面,new出来一个匿名类。这里,Key其实质是一个类的对象,Key包含两个Property:CategoryID、SupplierID。用g.Key.CategoryID可以遍历CategoryID的值。
var categories =from p in db.Productsgroup p by new { p.CategoryID, p.SupplierID } into gselect new {g.Key, g};
说明:按产品单价是否大于10分类。其结果分为两类,大于的是一类,小于及等于为另一类。
var categories =from p in db.Productsgroup p by new { Criterion = p.UnitPrice > 10 } into gselect g;
适用场景:用于判断集合中元素,进一步缩小范围。
说明:用于判断集合中是否有元素满足某一条件;不延迟。(若条件为空,则集合只要不为空就返回True,否则为False)。有2种形式,分别为简单形式和带条件形式。
var q =from c in db.Customerswhere !c.Orders.Any()select c;
生成SQL语句为:SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]FROM [dbo].[Customers] AS [t0]WHERE NOT (EXISTS(SELECT NULL AS [EMPTY]FROM [dbo].[Orders] AS [t1]WHERE [t1].[CustomerID] = [t0].[CustomerID]))
var q =from c in db.Categorieswhere c.Products.Any(p => p.Discontinued)select c;
生成SQL语句为:SELECT [t0].[CategoryID], [t0].[CategoryName], [t0].[Description], [t0].[Picture]FROM [dbo].[Categories] AS [t0]WHERE EXISTS(SELECT NULL AS [EMPTY]FROM [dbo].[Products] AS [t1]WHERE ([t1].[Discontinued] = 1) AND ([t1].[CategoryID] = [t0].[CategoryID]))
说明:用于判断集合中所有元素是否都满足某一条件;不延迟
var q =from c in db.Customerswhere c.Orders.All(o => o.ShipCity == c.City)select c;
说明:用于判断集合中是否包含有某一元素;不延迟。它是对两个序列进行连接操作的。
string[] customerID_Set = new string[] { "AROUT", "BOLID", "FISSA" };var q = (from o in db.Orderswhere customerID_Set.Contains(o.CustomerID)select o).ToList();
语句描述:查找"AROUT", "BOLID" 和 "FISSA" 这三个客户的订单。先定义了一个数组,在LINQ to SQL中使用Contains,数组中包含了所有的CustomerID,即返回结果中,所有的CustomerID都在这个集合内。也就是in。 你也可以把数组的定义放在LINQ to SQL语句里。比如:
var q = (from o in db.Orderswhere (new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID)select o).ToList();
Not Contains则取反:
var q = (from o in db.Orderswhere !(new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID)select o).ToList();
var order = (from o in db.Orderswhere o.OrderID == 10248select o).First();var q = db.Customers.Where(p => p.Orders.Contains(order)).ToList();foreach (var cust in q){foreach (var ord in cust.Orders){//do something}}
string[] cities = new string[] { "Seattle", "London", "Vancouver", "Paris" };var q = db.Customers.Where(p=>cities.Contains(p.City)).ToList();
总结一下这篇我们说明了以下语句:
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