LINQ to SQL语句小结之 Exists/In/Any/All/Contains(转载)
Exists/In/Any/All/Contains操作符
适用场景:用于判断集合中元素,进一步缩小范围。
Any
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。
1.简单形式:
仅返回没有订单的客户:
var q = from c in db.Customers where !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] ))
2.带条件形式:
仅返回至少有一种产品断货的类别:
var q = from c in db.Categories where 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]) )
All
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
1.带条件形式
var q = from c in db.Customers where c.Orders.All(o => o.ShipCity == c.City) select c;
语句描述:这个例子返回所有订单都运往其所在城市的客户或未下订单的客户。
Contains
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 oin db.Orders where 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.Orders where ( new string[] { "AROUT", "BOLID", "FISSA" }) .Contains(o.CustomerID) select o).ToList();
Not Contains 则取反:
var q = ( from o in db.Orders where !( new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID) select o).ToList();
1.包含一个对象:
var order = (from o in db.Orders where o.OrderID == 10248 select 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 } }
语句描述:这个例子使用Contain查找哪个客户包含OrderID为10248的订单。
2.包含多个值:
string[] cities = new string[] { "Seattle", "London", "Vancouver", "Paris" }; var q = db.Customers.Where(p=>cities.Contains(p.City)).ToList();
语句描述:这个例子使用Contains查找其所在城市为西雅图、伦敦、巴黎或温哥华的客户。