Sql LinqLambda 查询语句的区别

Posted 2020-09-30 刘家小生

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()