通过16道练习学习Linq和Lambda

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# [通过16道练习学习Linq和Lambda][16_Linq_Lambda] #

作者: [ Alex.Shi][Alex.Shi]  来源: [ 博客园][Link 1]  发布时间: 2010-09-08 14:54  阅读: 5561 次  推荐: 6   [ 原文链接][Link 2]   \[收藏\]

1、查询Student表中的所有记录的Sname、Ssex和Class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60到80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85，86或88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"女"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、以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、以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"和"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90的Sno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Sname、Cno和Degree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Sno、Cname和Degree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Sname、Cname和Degree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 }

主要参考文章链接：http://www.cnblogs.com/RuiLei/archive/2008/11/09/1329905.html

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[16_Linq_Lambda]: http://kb.cnblogs.com/page/73528/
[Alex.Shi]: http://www.cnblogs.com/alexworks/
[Link 1]: http://www.cnblogs.com/
[Link 2]: http://www.cnblogs.com/alexworks/articles/1618645.html
[Link 3]: http://kb.cnblogs.com/page/73531/
[Link 4]: http://kb.cnblogs.com/page/73486/