Linq练习题

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 }