一起谈.NET技术，通过16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

from t in Teachers.Distinct()

select t.DEPART

Teachers.Distinct().Select( t => t.DEPART)

3、 查询Student表的所有记录。

select * from student

select s

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

from s in Scores

where s.DEGREE >= 60 && s.DEGREE < 80

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

where (

new decimal[]{85,86,88}

).Contains(s.DEGREE)

Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE))

Not in

where !(

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'

where s.CLASS == "95031"

|| s.CLASS == "女"

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

orderby s.CLASS descending

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)

orderby s.DEGREE descending

orderby s.CNO ascending

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'

( from s in Students

).Count()

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))

(

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

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

                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'

where s.CNO == "3-105"

select s.DEGREE

).Average()

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

where s.CNO.StartsWith("3")

group s by s.CNO

into cc

where cc.Count() >= 5

select cc.Average( c => c.DEGREE)

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

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

Scores.GroupBy (s => s.SNO)

.Where (ss => ((ss.Min (cc => cc.DEGREE) > 70) && (ss.Max (cc => cc.DEGREE) < 90)))

.Select ( ss => new {

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

join sc in Scores

on s.SNO equals sc.SNO

sc.CNO,

sc.DEGREE

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

on c.CNO equals sc.CNO

sc.SNO,c.CNAME,sc.DEGREE

Courses.Join ( Scores, c => c.CNO,

sc => sc.CNO,

(c, sc) => new

SNO = sc.SNO,

CNAME = c.CNAME,

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

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