LINQ查询

1.查询语法与方法语法

查询语法：

var queryResults =

                                from n in names

                                where n.StartsWith("S")

                                select n;

方法语法：

var queryResults = names.Where(n=>n.StartsWith("S"));

2.查询排序

默认升序，添加descending关键字则为降序

var queryResults =

                                from n in names

                                where n.StartsWith("S")

                                orderby n descending

                                select n;

3.聚合运算

var queryResults= 

                              from n in numbers

                              where n>1000

                              select n;

var maxNum=queryResult.Max();

4.投影:在查询中创建新类型

List<Customer> customers=new List<Customer>{new Customer{ID="A",City="Beijing",Country="China",Region="Asia",Sales=9}}

var queryResults=

                              from c in customers

                              where c.Region="Asia"

                              select new {c.City,c.Country,c.Sales};

5.Any()和All()方法

bool anyUSA=customers.Any(c=>c.Country=="USA");

bool allAsia=customers.All(c=>c.Region=="Asia");

6.多级排序

var queryResults=                                                                   或     var queryResults=customers.OrderBy(c=>c.Region)

                             from c in customers                                                                        .ThenBy(c=>c.Country)

                             orderby c.Region,c.Country,c.City                                                  .ThenBy(c=>c.City)

                             select new {c.ID,c.Region,c.Country,c.City};                                  .Select(c=>new {c.ID,c.Region,c.Country,c.City})

7.组合查询

var queryResults=

                             from c in customers

                             group c by c.Region into cg

                             select new {TotalSales=cg.Sum(c=>c.Sales),Region=cg.Key};

8.Take()和Skip()方法

queryResults.Take(n):取前n个数据                  queryResults.Skip(n):跳过前n个数据

9.Frist()和FristOrDefalt()方法

Frist()如果没有满足条件的会报错，而FristOrDefalt不会

10.集运算Intersect(),Except(),Union()方法

11.多表查询join...on...

var queryResults=

                             from c in customers

                             join o in orders on c.Id equals o.Id