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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()
10.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)
11.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()
12.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")
13.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")
14. 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()
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原文地址：http://www.cnblogs.com/xiaz/p/5324328.html
