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linq入门 linq介绍

2012/8/26 17:28:24

一  Linq 定义
Linq 全称Language Integrated Query,它由Linq to xml、Linq to objects、Linq to dataset、Linq to entities等组成。它提供了C#编程语言中的查询语法,可以使用相同的语法访问不同的数据源。


二、Lambda表达式


什么是lambda表达式?Lambda 表达式是一种匿名函数,它可以包含表达式和语句,并且可用于创建委托或
表达式目录树类型。我们使用lambda表达式可以帮助我们编写精简和紧凑的代码,许多操作中允许自定义排序和过
滤的函数,在.NET2.0的时候通常使用委托函数来实现,在.NET3.5可以使用lambda表达式。
现在举例说明lambda表达式:  Func<int,int> addOne= item=> item+1 ,其中操作符 “=>”读作“Goes to”,
可以理解为操作符左边的是函数的参数,操作符右边是函数体内容。上面我们定义的lambda表达式等同于函数如下:
 
1 int addOne(int item)
2 {
3  return item+1;
4 }
 
那么什么样的表达式才是合法的lambda表达式呢?
1. lambda表达式可以是多个参数。 如:  (item1,item2)=>item1+item2;
2. lambda表达式可以是0个参数。 如: ()=>"csharp";
3. lambda表达式可以显示指定参数类型。 (int item1,string item2)=>item1+item2;
4. lambda表达式函数体可以使用多条语句. (item1)=>{string ret="hello"+item1;return ret;};


三、linq 语法

  1. ·First - 返回集合中的第一个元素;不延迟  
  2.     ·FirstOrDefault - 返回集合中的第一个元素(如果没有则返回默认值);不延迟  
  3.     ·Last - 返回集合中的最后一个元素;不延迟  
  4.     ·LastOrDefault - 返回集合中的最后一个元素(如果没有则返回默认值)  
  5.     ·ElementAt - 返回集合中指定索引的元素;不延迟  
  6.     ·ElementAtOrDefault - 返回集合中指定索引的元素(如果没有则返回默认值);不延迟  
  7.     ·Contains - 判断集合中是否包含有某一元素;不延迟  
  8.     ·Any - 判断集合中是否有元素满足某一条件;不延迟  
  9.     ·All - 判断集合中是否所有元素都满足某一条件;不延迟  
  10.     ·Count - 返回集合中的元素个数,返回int;不延迟  
  11.     ·LongCount - 返回集合中的元素个数,返回long;不延迟  
  12.     ·Sum - 集合应为数字类型集合,求其和;不延迟  
  13.     ·Min - 返回集合的最小值;不延迟  
  14.     ·Max - 返回集合的最大值;不延迟  
  15.     ·Average - 集合应为数字类型集合,求其平均值;不延迟  
  16.     ·Aggregate - 根据输入的表达式获取一个聚合值;不延迟  
  17.     ·Cast - 将集合转换为强类型集合;延迟  
  18.     ·DefaultIfEmpty - 查询结果为空则返回默认值;延迟  
  19.     ·SequenceEqual - 判断两个集合是否相同;不延迟  
  20.     ·OfType - 过滤集合中的指定类型;延迟  
  21.     ·ToArray - 将集合转换为数组;不延迟  
  22.     ·ToList - 将集合转换为List<T>集合;不延迟  
  23.     ·ToDictionary - 将集合转换为<K, V>集合;不延迟  
四、linq常见任务示例

1 查询Student表中的所有记录的SnameSsexClass列。 

select sname,ssex,class from student Linq:  from s in Students
    select new {  s.SNAME,  s.SSEX,  s.CLASS  } Lambda:  Students.Select( s => new {
        SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
    })


2 查询教师所有的单位即不重复的Depart列。
 

select distinct depart from teacher Linq:  from t in Teachers.Distinct()
    select t.DEPART Lambda:
    Teachers.Distinct().Select( t => t.DEPART)

 

3 查询Student表的所有记录。
 

select * from student Linq:  from s in Students
    select s Lambda:  Students.Select( s => s)

 

4 查询Score表中成绩在6080之间的所有记录。 

select * from score where degree between 60 and 80 Linq:  from s in Scores
    where s.DEGREE >= 60 && s.DEGREE < 80
    select s Lambda:  Scores.Where(   s => (
                s.DEGREE >= 60 && s.DEGREE < 80 
             )
    )

 

5 查询Score表中成绩为858688的记录。 

select * from score where degree in (85,86,88) Linq: In
    from s in Scores
    where (
            new decimal[]{85,86,88}
          ).Contains(s.DEGREE)
    select s Lambda:  Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE)) Not in
    from s in Scores
    where !(
            new decimal[]{85,86,88}
          ).Contains(s.DEGREE)
    select s Lambda:  Scores.Where( s => !(new Decimal[]{85,86,88}.Contains(s.DEGREE))) 

  Any()应用:双表进行Any时,必须是主键为(String)
  CustomerDemographics CustomerTypeID(String)
  CustomerCustomerDemos (CustomerID CustomerTypeID)(String)
  一个主键与二个主建进行Any(或者是一对一关键进行Any)不可,以二个主键于与一个主键进行Any

from e in CustomerDemographics
where !e.CustomerCustomerDemos.Any()
select e

from c in Categories
where !c.Products.Any()
select c

 

 

6 查询Student表中"95031"班或性别为""的同学记录。 

select * from student where class ='95031' or ssex= N'女' Linq:
    from s in Students
    where s.CLASS == "95031" 
       || s.CLASS == "女"
    select s Lambda:  Students.Where(s => ( s.CLASS == "95031" || s.CLASS == "女"))

 

7 Class降序查询Student表的所有记录。

select * from student order by Class DESC Linq:  from s in Students
    orderby s.CLASS descending
    select s Lambda:  Students.OrderByDescending(s => s.CLASS)

 

8 Cno升序、Degree降序查询Score表的所有记录。 

select * from score order by Cno ASC,Degree DESC Linq:(这里Cno ASC在linq中要写在最外面)  from s in Scores
    orderby s.DEGREE descending
    orderby s.CNO ascending 
    select s Lambda:  Scores.OrderByDescending( s => s.DEGREE)
          .OrderBy( s => s.CNO)

 

9 查询"95031"班的学生人数。

select count(*) from student where class = '95031' Linq:
    (    from s in Students
        where s.CLASS == "95031"
        select s  ).Count() Lambda:  Students.Where( s => s.CLASS == "95031" )
                .Select( s => s)
                    .Count()

 

10、查询Score表中的最高分的学生学号和课程号。 

select distinct s.Sno,c.Cno from student as s,course as c ,score as sc 
where s.sno=(select sno from score where degree = (select max(degree) from score))
and c.cno = (select cno from score where degree = (select max(degree) from score)) Linq:  (  from s in Students
        from c in Courses
        from sc in Scores
        let maxDegree = (from sss in Scores
                        select sss.DEGREE
                        ).Max()
        let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).Single().ToString()
        let cno = (from ssss in Scores
                where ssss.DEGREE == maxDegree
                select ssss.CNO).Single().ToString()
        where s.SNO == sno && c.CNO == cno
        select new {
            s.SNO,
            c.CNO
        }
    ).Distinct()

  操作时问题?执行时报错: where s.SNO == sno(这行报出来的) 运算符"=="无法应用于"string""System.Linq.IQueryable<string>"类型的操作数
  解决:
  原:let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).ToString()
      Queryable().Single()
返回序列的唯一元素;如果该序列并非恰好包含一个元素,则会引发异常。
  解:let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).Single().ToString()
 

11、查询'3-105'号课程的平均分。

select avg(degree) from score where cno = '3-105' Linq:
    (
        from s in Scores
        where s.CNO == "3-105"
        select s.DEGREE  ).Average() Lambda:  Scores.Where( s => s.CNO == "3-105")
            .Select( s => s.DEGREE)
                .Average()

 



12、查询Score表中至少有5名学生选修的并以3开头的课程的平均分数。

select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5 Linq:
        from s in Scores
        where s.CNO.StartsWith("3")
        group s by s.CNO
        into cc
        where cc.Count() >= 5
        select cc.Average( c => c.DEGREE) Lambda:  Scores.Where( s => s.CNO.StartsWith("3") )
            .GroupBy( s => s.CNO )
              .Where( cc => ( cc.Count() >= 5) )
                .Select( cc => cc.Average( c => c.DEGREE) )
  Linq: SqlMethod
  like
也可以这样写:
    s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")



13、查询最低分大于70,最高分小于90Sno列。

select sno from score group by sno having min(degree) > 70 and max(degree) < 90 Linq:  from s in Scores
    group s by s.SNO
    into ss
    where ss.Min(cc => cc.DEGREE) > 70 && ss.Max( cc => cc.DEGREE) < 90
    select new
    {
        sno = ss.Key  } Lambda:  Scores.GroupBy (s => s.SNO)
               .Where (ss => ((ss.Min (cc => cc.DEGREE) > 70) && (ss.Max (cc => cc.DEGREE) < 90)))
                   .Select ( ss => new {
                                        sno = ss.Key
                                     })

 



14、查询所有学生的SnameCnoDegree列。

select s.sname,sc.cno,sc.degree from student as s,score as sc where s.sno = sc.sno Linq:  from s in Students
    join sc in Scores
    on s.SNO equals sc.SNO
    select new  {  s.SNAME,  sc.CNO,  sc.DEGREE  } Lambda:  Students.Join(Scores, s => s.SNO,
                          sc => sc.SNO, 
                          (s,sc) => new{
                                            SNAME = s.SNAME,
                                            CNO = sc.CNO,
                                            DEGREE = sc.DEGREE
                                          })

 



15、查询所有学生的SnoCnameDegree列。

select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno Linq:  from c in Courses
    join sc in Scores
    on c.CNO equals sc.CNO
    select new  {  sc.SNO,c.CNAME,sc.DEGREE  } Lambda:  Courses.Join ( Scores, c => c.CNO, 
                             sc => sc.CNO, 
                             (c, sc) => new  
                                        {
                                            SNO = sc.SNO, 
                                            CNAME = c.CNAME, 
                                            DEGREE = sc.DEGREE
                                        })


16、查询所有学生的SnameCnameDegree列。

select s.sname,c.cname,sc.degree from student as s,course as c,score as sc where s.sno = sc.sno and c.cno = sc.cno Linq:  from s in Students
    from c in Courses
    from sc in Scores
    where s.SNO == sc.SNO && c.CNO == sc.CNO
    select new { s.SNAME,c.CNAME,sc.DEGREE }