通过16道练习学习Linq和Lambda

1、查询Student表中的所有记录的Sname、Ssex和Class列。 

select sname,ssex,class from studentLinq:    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 teacherLinq:    from t in Teachers.Distinct()    select t.DEPARTLambda:    Teachers.Distinct().Select( t => t.DEPART)

 

3、查询Student表的所有记录。
 

select * from studentLinq:    from s in Students    select sLambda:    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 80Linq:    from s in Scores    where s.DEGREE >= 60 && s.DEGREE < 80    select sLambda:    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 sLambda:    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 sLambda:    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 CustomerDemographicswhere !e.CustomerCustomerDemos.Any()select efrom c in Categorieswhere !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 sLambda:    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 DESCLinq:    from s in Students    orderby s.CLASS descending    select sLambda:    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 DESCLinq:(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 sLambda:    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(*)>=5Linq:        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) < 90Linq:    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.snoLinq:    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.cnoLinq:    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.cnoLinq:    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 }