Linq 实例

1.分页

var 分页 = (from c in ctx.Customers select c).Skip(10).Take(10);

2.分组

1）一般分组

 
//根据顾客的国家分组，查询顾客数大于5的国家名和顾客数
var 一般分组 = from c in ctx.Customers

                   group c by c.Country into g

                   where g.Count() > 5

                   orderby g.Count() descending

                   select new

                   {

                       国家 = g.Key,

                       顾客数 = g.Count()

                   };

2）匿名类型分组 (根据国家和城市分组，查询顾客覆盖的国家和城市)

 var 匿名类型分组 = from c in ctx.Customers

                     group c by new { c.City, c.Country } into g

                     orderby g.Key.Country, g.Key.City

                     select new

                     {

                         国家 = g.Key.Country,

                         城市 = g.Key.City

                     };

3）按条件分组

//按照是否超重条件分组，分别查询订单数量
var 按照条件分组 = from o in ctx.Orders

                     group o by new { 条件 = o.Freight > 100 } into g

                     select new

                     {

                         数量 = g.Count(),

                         是否超重 = g.Key.条件 ? "是" : "否"

                     };

3distinct

//查询顾客覆盖的国家
var 过滤相同项 = (from c in ctx.Customers orderby c.Country select c.Country).Distinct();

4 union

//查询城市是A打头和城市包含A的顾客并按照顾客名字排序
var 连接并且过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Union

            (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

5 concat

//查询城市是A打头和城市包含A的顾客并按照顾客名字排序，相同的顾客信息不会过滤
var 连接并且不过滤相同项 = (from c in ctx.Customers where c.City.Contains("A") select c).Concat

            (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

6 取相交项

//查询城市是A打头的顾客和城市包含A的顾客的交集，并按照顾客名字排序
var 取相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Intersect

            (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

7 排除相交项

//查询城市包含A的顾客并从中删除城市以A开头的顾客，并按照顾客名字排序
var 排除相交项 = (from c in ctx.Customers where c.City.Contains("A") select c).Except

            (from c in ctx.Customers where c.ContactName.StartsWith("A") select c).OrderBy(c => c.ContactName);

8 子查询 

//查询订单数超过5的顾客信息
var 子查询 = from c in ctx.Customers

                   where

                       (from o in ctx.Orders group o by o.CustomerID into o where o.Count() > 5 select o.Key).Contains(c.CustomerID)

                   select c;

9 in操作

//查询指定城市中的客户
 var in操作 = from c in ctx.Customers

                    where new string[] { "Brandenburg", "Cowes", "Stavern" }.Contains(c.City)

                    select c;