[转]通过15道练习学习Linq和Lambda

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)

 

In

Linq:

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

Linq:

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)))

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") //like也可以这样写 s.CNO.StartsWith("3") OR SqlMethods.Like(s.CNO,"%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) )

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 }

    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

转载自:http://www.cnblogs.com/RuiLei/archive/2008/11/09/1329905.html