SQL、LINQ、Lambda三种方式比较

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

sql:　　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 

from t in Teachers.Distinct()     select t.DEPART 

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 

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 

from s in Scores     where s.DEGREE >= 60 && s.DEGREE < 80     select s 

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) 

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'

from s in Students     where s.CLASS == "95031"        || s.CLASS == "女" select s 

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 

from s in Students     orderby s.CLASS descending     select s 

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 
(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 
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' 

(    from s in Students where s.CLASS == "95031"  select s     ).Count() 

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)) 

( 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' 

(         

　　from s in Scores         

　　where s.CNO == "3-105"         

　　select s.DEGREE     

).Average() 

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 

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) 

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 

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     

} 

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 

from s in Students     join sc in Scores     on s.SNO equals sc.SNO     select new     {         s.SNAME,         sc.CNO,         sc.DEGREE     } 

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 

from c in Courses     join sc in Scores     on c.CNO equals sc.CNO     select new     {         sc.SNO,c.CNAME,sc.DEGREE     }

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 

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 }