sql 、linq、lambda 查詢語句的區別

LINQ的書寫格式如下： 　
　from 臨時變數 in 集合對象或資料庫對象 　
　where 條件表達式 　　
[order by條件] 　　
select 臨時變數中被查詢的值 　
　[group by 條件]

Lambda表達式的書寫格式如下：

(參數列表) => 表達式或者語句塊

其中： 參數個數：可以有多個參數，一個參數，或者無參數。

參數類型：可以隱式或者顯式定義。

表達式或者語句塊：這部分就是我們平常寫函數的實現部分（函數體）。

1.查詢全部

實例 Code 
查詢Student表的所有記錄。
select * from student
Linq:
    from s in Students
    select s
Lambda:
    Students.Select( s => s)

2 按條件查詢全部：

實例 Code 
 查詢Student表中的所有記錄的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 去掉重覆的

實例 Code 
查詢教師所有的單位即不重覆的Depart列。
select distinct depart from teacher
Linq:
    from t in Teachers.Distinct()
    select t.DEPART
Lambda:
    Teachers.Distinct().Select( t => t.DEPART)

4.連接查詢 between and

實例 Code 
查詢Score表中成績在60到80之間的所有記錄。
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.在範圍內篩選 In

實例 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 條件過濾

實例 Code 
查詢Student表中"95031"班或性別為"女"的同學記錄。
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

實例 Code 
以Class降序查詢Student表的所有記錄。
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（）行數查詢

實例 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()

10.avg()平均

實例 Code 
查詢'3-105'號課程的平均分。
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.子查詢

實例  Code 
查詢Score表中的最高分的學生學號和課程號。
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.分組 過濾

實例 Code 
查詢Score表中至少有5名學生選修的並以3開頭的課程的平均分數。
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也可以這樣寫:
    s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")

13.分組

實例 Code 
查詢Score表中至少有5名學生選修的並以3開頭的課程的平均分數。
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也可以這樣寫:
    s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")

14. 多表查詢

實例 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()