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通过16道练习学习Linq和Lambda

作者: Alex.Shi  来源: 博客园  发布时间: 2010-09-08 14:54  阅读: 7728 次  推荐: 9   原文链接   [收藏]  

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 }

 

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

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