sql 、linq、lambda 查询语句的区别

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

1 查询Student表的所有记录。
2 select * from student
3 Linq:
4     from s in Students
5     select s
6 Lambda:
7     Students.Select( s => s)

2 按条件查询全部：

1  查询Student表中的所有记录的Sname、Ssex和Class列。
2 select sname,ssex,class from student
3 Linq:
4     from s in Students
5     select new {
6         s.SNAME,
7         s.SSEX,
8         s.CLASS
9     }
10 Lambda:
11     Students.Select( s => new {
12         SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
13     })

3.distinct 去掉重复的

1 查询教师所有的单位即不重复的Depart列。
2 select distinct depart from teacher
3 Linq:
4     from t in Teachers.Distinct()
5     select t.DEPART
6 Lambda:
7     Teachers.Distinct().Select( t => t.DEPART)

4.连接查询 between and

1 查询Score表中成绩在60到80之间的所有记录。
2 select * from score where degree between 60 and 80
3 Linq:
4     from s in Scores
5     where s.DEGREE >= 60 && s.DEGREE < 80
6     select s
7 Lambda:
8     Scores.Where(
9         s => (
10                 s.DEGREE >= 60 && s.DEGREE < 80
11              )
12     )

5.在范围内筛选 In

1 select * from score where degree in (85,86,88)
2 Linq:
3     from s in Scores
4     where (
5             new decimal[]{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

1 查询Student表中"95031"班或性别为"女"的同学记录。
2 select * from student where class ='95031' or ssex= N'女'
3 Linq:
4     from s in Students
5     where s.CLASS == "95031"
6        || s.CLASS == "女"
7     select s
8 Lambda:
9     Students.Where(s => ( s.CLASS == "95031" || s.CLASS == "女"))

7.排序

1 以Class降序查询Student表的所有记录。
2 select * from student order by Class DESC
3 Linq:
4     from s in Students
5     orderby s.CLASS descending
6     select s
7 Lambda:
8     Students.OrderByDescending(s => s.CLASS)

8.count（）行数查询

1 select count(*) from student where class = '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

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

11.子查询

1 查询Score表中的最高分的学生学号和课程号。
2 select distinct s.Sno,c.Cno from student as s,course as c ,score as sc
3 where s.sno=(select sno from score where degree = (select max(degree) from score))
4 and c.cno = (select cno from score where degree = (select max(degree) from score))
5 Linq:
6     (
7         from s in Students
8         from c in Courses
9         from sc in Scores
10         let maxDegree = (from sss in Scores
11                         select sss.DEGREE
12                         ).Max()
13         let sno = (from ss in Scores
14                 where ss.DEGREE == maxDegree
15                 select ss.SNO).Single().ToString()
16         let cno = (from ssss in Scores
17                 where ssss.DEGREE == maxDegree
18                 select ssss.CNO).Single().ToString()
19         where s.SNO == sno && c.CNO == cno
20         select new {
21             s.SNO,
22             c.CNO
23         }
24     ).Distinct()

12.分组 过滤

1 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。
2 select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3 Linq:
4         from s in Scores
5         where s.CNO.StartsWith("3")
6         group s by s.CNO
7         into cc
8         where cc.Count() >= 5
9         select cc.Average( c => c.DEGREE)
10 Lambda:
11     Scores.Where( s => s.CNO.StartsWith("3") )
12             .GroupBy( s => s.CNO )
13               .Where( cc => ( cc.Count() >= 5) )
14                 .Select( cc => cc.Average( c => c.DEGREE) )
15 Linq: SqlMethod
16 like也可以这样写:
17     s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")

13.分组

1 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。
2 select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3 Linq:
4         from s in Scores
5         where s.CNO.StartsWith("3")
6         group s by s.CNO
7         into cc
8         where cc.Count() >= 5
9         select cc.Average( c => c.DEGREE)
10 Lambda:
11     Scores.Where( s => s.CNO.StartsWith("3") )
12             .GroupBy( s => s.CNO )
13               .Where( cc => ( cc.Count() >= 5) )
14                 .Select( cc => cc.Average( c => c.DEGREE) )
15 Linq: SqlMethod
16 like也可以这样写:
17     s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")

14. 多表查询

1 select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno
2 Linq:
3     from c in Courses
4     join sc in Scores
5     on c.CNO equals sc.CNO
6     select new
7     {
8         sc.SNO,c.CNAME,sc.DEGREE
9     }
10 Lambda:
11     Courses.Join ( Scores, c => c.CNO,
12                              sc => sc.CNO,
13                              (c, sc) => new
14                                         {
15                                             SNO = sc.SNO,
16                                             CNAME = c.CNAME,
17                                             DEGREE = sc.DEGREE
18                                         })
19                 .Average()

（在此，特别感谢网友“王者之舞”所提供的实例）

标签: LINQ与SQL