浅谈sql 、linq、lambda 查询语句的区别

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。

1 select * from student
2 Linq:
3     from s in Students
4     select s
5 Lambda:
6     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。

 1  select sname,ssex,class from student
 2  Linq:
 3      from s in Students
 4      select new {
 5          s.SNAME,
 6          s.SSEX,
 7          s.CLASS
 8      }
 9 Lambda:
10     Students.Select( s =>new {
11         SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
12     })

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。

 1 select * from score where degree between 60 and 80
 2  Linq:
 3      from s in Scores
 4  where s.DEGREE >=60&& s.DEGREE <80
 5      select s
 6  Lambda:
 7      Scores.Where(
 8          s => (
 9                 s.DEGREE >=60&& s.DEGREE <80
10              )
11     )

5.在范围内筛选 In

实例 Code

1 select * from score where degree in (85,86,88)
2 Linq:
3     from s in Scores
4 where (
5 newdecimal[]{85,86,88}
6           ).Contains(s.DEGREE)
7     select s
8 Lambda:
9     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。

1 select * from student whereclass='95031' or ssex= N'女'
2 Linq:
3     from s in Students
4 where s.CLASS =="95031"
5 || s.CLASS =="女"
6     select s
7 Lambda:
8     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。

1 select * from student order by Class DESC
2 Linq:
3     from s in Students
4     orderby s.CLASS descending
5     select s
6 Lambda:
7     Students.OrderByDescending(s => s.CLASS)

8.count（）行数查询

实例 Code

 1 select count(*) from student whereclass='95031'
 2  Linq:
 3      (    from s in Students
 4  where s.CLASS =="95031"
 5          select s
 6      ).Count()
 7  Lambda:
 8      Students.Where( s => s.CLASS =="95031" )
 9                  .Select( s => s)
10                     .Count()

10.avg()平均

实例 Code

查询'3-105'号课程的平均分。

 1 select avg(degree) from score where cno ='3-105'
 2  Linq:
 3      (
 4          from s in Scores
 5  where s.CNO =="3-105"
 6          select s.DEGREE
 7      ).Average()
 8  Lambda:
 9     Scores.Where( s => s.CNO =="3-105")
10             .Select( s => s.DEGREE)

11.子查询

实例 Code

查询Score表中的最高分的学生学号和课程号。

 1  select distinct s.Sno,c.Cno from student as s,course as c ,score as sc
 2  where s.sno=(select sno from score where degree = (select max(degree) from score))
 3  and c.cno = (select cno from score where degree = (select max(degree) from score))
 4  Linq:
 5      (
 6          from s in Students
 7          from c in Courses
 8          from sc in Scores
 9         let maxDegree = (from sss in Scores
10                         select sss.DEGREE
11                         ).Max()
12         let sno = (from ss in Scores
13 where ss.DEGREE == maxDegree
14                 select ss.SNO).Single().ToString()
15         let cno = (from ssss in Scores
16 where ssss.DEGREE == maxDegree
17                 select ssss.CNO).Single().ToString()
18 where s.SNO == sno && c.CNO == cno
19         select new {
20             s.SNO,
21             c.CNO
22         }
23     ).Distinct()

12.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")

13.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")

14. 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()