一 Linq 定义
Linq 全称Language Integrated Query,它由Linq to xml、Linq to objects、Linq to dataset、Linq to entities等组成。它提供了C#编程语言中的查询语法,可以使用相同的语法访问不同的数据源。
什么是lambda表达式?Lambda 表达式是一种匿名函数,它可以包含表达式和语句,并且可用于创建委托或
表达式目录树类型。我们使用lambda表达式可以帮助我们编写精简和紧凑的代码,许多操作中允许自定义排序和过
滤的函数,在.NET2.0的时候通常使用委托函数来实现,在.NET3.5可以使用lambda表达式。
现在举例说明lambda表达式: Func<int,int> addOne= item=> item+1 ,其中操作符 “=>”读作“Goes to”,
可以理解为操作符左边的是函数的参数,操作符右边是函数体内容。上面我们定义的lambda表达式等同于函数如下:
1 int addOne(int item)
2 {
3 return item+1;
4 }
那么什么样的表达式才是合法的lambda表达式呢?
1. lambda表达式可以是多个参数。 如: (item1,item2)=>item1+item2;
2. lambda表达式可以是0个参数。 如: ()=>"csharp";
3. lambda表达式可以显示指定参数类型。 (int item1,string item2)=>item1+item2;
4. lambda表达式函数体可以使用多条语句. (item1)=>{string ret="hello"+item1;return ret;};
三、linq 语法
- ·First - 返回集合中的第一个元素;不延迟
- ·FirstOrDefault - 返回集合中的第一个元素(如果没有则返回默认值);不延迟
- ·Last - 返回集合中的最后一个元素;不延迟
- ·LastOrDefault - 返回集合中的最后一个元素(如果没有则返回默认值)
- ·ElementAt - 返回集合中指定索引的元素;不延迟
- ·ElementAtOrDefault - 返回集合中指定索引的元素(如果没有则返回默认值);不延迟
- ·Contains - 判断集合中是否包含有某一元素;不延迟
- ·Any - 判断集合中是否有元素满足某一条件;不延迟
- ·All - 判断集合中是否所有元素都满足某一条件;不延迟
- ·Count - 返回集合中的元素个数,返回int;不延迟
- ·LongCount - 返回集合中的元素个数,返回long;不延迟
- ·Sum - 集合应为数字类型集合,求其和;不延迟
- ·Min - 返回集合的最小值;不延迟
- ·Max - 返回集合的最大值;不延迟
- ·Average - 集合应为数字类型集合,求其平均值;不延迟
- ·Aggregate - 根据输入的表达式获取一个聚合值;不延迟
- ·Cast - 将集合转换为强类型集合;延迟
- ·DefaultIfEmpty - 查询结果为空则返回默认值;延迟
- ·SequenceEqual - 判断两个集合是否相同;不延迟
- ·OfType - 过滤集合中的指定类型;延迟
- ·ToArray - 将集合转换为数组;不延迟
- ·ToList - 将集合转换为List<T>集合;不延迟
- ·ToDictionary - 将集合转换为<K, V>集合;不延迟
1、 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 })
2、 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)
3、 查询Student表的所有记录。
select * from student Linq: from s in Students select s Lambda: Students.Select( s => s)
4、 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、 查询Score表中成绩为85,86或88的记录。
select * from score where degree in (85,86,88) Linq: In 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)) Not in 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)))
Any()应用:双表进行Any时,必须是主键为(String)
CustomerDemographics
CustomerTypeID(String)
CustomerCustomerDemos (CustomerID
CustomerTypeID)(String)
一个主键与二个主建进行Any(或者是一对一关键进行Any)不可,以二个主键于与一个主键进行Any
from e in CustomerDemographics
where !e.CustomerCustomerDemos.Any()
select e
from c in Categories
where !c.Products.Any()
select c
6、 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、 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、 以Cno升序、Degree降序查询Score表的所有记录。
select * from score order by Cno ASC,Degree DESC Linq:(这里Cno ASC在linq中要写在最外面) from s in Scores orderby s.DEGREE descending orderby s.CNO ascending select s Lambda: Scores.OrderByDescending( s => s.DEGREE) .OrderBy( s => s.CNO)
9、 查询"95031"班的学生人数。
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、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()
操作时问题?执行时报错: where s.SNO == sno(这行报出来的) 运算符"=="无法应用于"string"和"System.Linq.IQueryable<string>"类型的操作数
解决:
原:let
sno = (from ss in Scores
where ss.DEGREE ==
maxDegree
select ss.SNO).ToString()
Queryable().Single()返回序列的唯一元素;如果该序列并非恰好包含一个元素,则会引发异常。
解:let
sno = (from ss in Scores
where ss.DEGREE ==
maxDegree
select
ss.SNO).Single().ToString()
11、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) .Average()
12、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) )
like也可以这样写:
s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")
13、查询最低分大于70,最高分小于90的Sno列。
select sno from score group by sno having min(degree) > 70 and max(degree) < 90 Linq: from s in Scores group s by s.SNO into ss where ss.Min(cc => cc.DEGREE) > 70 && ss.Max( cc => cc.DEGREE) < 90 select new { sno = ss.Key } Lambda: Scores.GroupBy (s => s.SNO) .Where (ss => ((ss.Min (cc => cc.DEGREE) > 70) && (ss.Max (cc => cc.DEGREE) < 90))) .Select ( ss => new { sno = ss.Key })
14、查询所有学生的Sname、Cno和Degree列。
select s.sname,sc.cno,sc.degree from student as s,score as sc where s.sno = sc.sno Linq: from s in Students join sc in Scores on s.SNO equals sc.SNO select new { s.SNAME, sc.CNO, sc.DEGREE } Lambda: Students.Join(Scores, s => s.SNO, sc => sc.SNO, (s,sc) => new{ SNAME = s.SNAME, CNO = sc.CNO, DEGREE = sc.DEGREE })
15、查询所有学生的Sno、Cname和Degree列。
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 })
16、查询所有学生的Sname、Cname和Degree列。
select s.sname,c.cname,sc.degree from student as s,course as c,score as sc where s.sno = sc.sno and c.cno = sc.cno Linq: from s in Students from c in Courses from sc in Scores where s.SNO == sc.SNO && c.CNO == sc.CNO select new { s.SNAME,c.CNAME,sc.DEGREE }