linq与lambda 常用查询语句写法对比

linq与lambda 常用查询语句写法对比

原文地址：http://www.cnblogs.com/xust/articles/2700244.html

LINQ的书写格式如下： 　
　from 临时变量 in 集合对象或数据库对象 　
　where 条件表达式 　　
[order by条件] 　　
select 临时变量中被查询的值 　
　[group by 条件]

Lambda表达式的书写格式如下：

(参数列表) => 表达式或者语句块

其中： 参数个数：可以有多个参数，一个参数，或者无参数。

参数类型：可以隐式或者显式定义。

表达式或者语句块：这部分就是我们平常写函数的实现部分（函数体）。 

1.查询全部

实例 Code 
 查询Student表的所有记录。
 select * from student
 Linq:
     from s in Students
     select s
 Lambda:
     Students.Select( s => s)

2 按条件查询全部：

实例 Code 
  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
     })

3.distinct 去掉重复的

实例 Code 
 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)

4.连接查询 between and

实例 Code 
 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.在范围内筛选 In

实例 Code 
 select * from score where degree in (85,86,88)
 Linq:
     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))

6.or 条件过滤

实例 Code 
 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.排序

实例 Code 
 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.count（）行数查询

实例 Code 
 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()

9.avg()平均

实例 Code 
 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)

10.子查询

实例  Code 
 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()

11.分组 过滤

实例 Code 
 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")

12.分组

实例 Code 
 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. 多表查询

实例 Code 
 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
                                         })
                 .Average()

--自己的例子

IList<DCKeyword> mDCKeywordJoinLst = mDCKeywordLst.Join(
mDCKeywordImportants,
word => word.keyword_id,
impWord => impWord.keyword_id,
(word, impWord) => word
).ToList(); //重点关键词