sql 、linq、lambda 总结

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

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：

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

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

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

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

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

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)
 count（）行数查询

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.avg()平均

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)

11.子查询

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()

12.分组 过滤

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

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

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()