linq lanbda表达式的用法

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 }