LINQ to SQL语句(7)之Exists/In/Any/All/Contains

2016-12-22 12:09:14来源:cnblogs.com作者:可乐•雪碧•咖啡•茶人点击

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;
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 o in 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。

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