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 }

 

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