LINQ To SQL && Lambda 使用方法小结

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 }