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
1 查询Student表的所有记录。
select * from student
Linq:
from s in Students
select s
Lambda:
Students.Select( s => s)
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
})
.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)
.连接查询 between and
查询Score表中成绩在0到0之间的所有记录。
select * from score where degree between 0 and 0
Linq:
from s in Scores
where s.DEGREE >= 0 && s.DEGREE < 0
select s
Lambda:
Scores.Where(
s => (
s.DEGREE >= 0 && s.DEGREE < 0
)
)
.在范围内筛选 In
select * from score where degree in (,,)
Linq:
from s in Scores
where (
new decimal[]{,,}
).Contains(s.DEGREE)
select s
Lambda:
Scores.Where( s => new Decimal[] {,,}.Contains(s.DEGREE))
.or 条件过滤
查询Student表中"0"班或性别为"女"的同学记录。
select * from student where class ='0' or ssex= N'女'
Linq:
from s in Students
where s.CLASS == "0"
|| s.CLASS == "女"
select s
Lambda:
Students.Where(s => ( s.CLASS == "0" || s.CLASS == "女"))
.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 = '0'
Linq:
( from s in Students
where s.CLASS == "0"
select s
).Count()
Lambda:
Students.Where( s => s.CLASS == "0" )
.Select( s => s)
.Count()
.avg()平均
查询'-'号课程的平均分。
select avg(degree) from score where cno = '-'
Linq:
(
from s in Scores
where s.CNO == "-"
select s.DEGREE
).Average()
Lambda:
Scores.Where( s => s.CNO == "-")
.Select( s => s.degree)
或者：Scores.Average(p => p.degree);
.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
0 select new {
s.SNO,
c.CNO
}
).Distinct()
.分组 过滤
查询Score表中至少有名学生选修的并以开头的课程的平均分数。
select avg(degree) from score where cno like '%' group by Cno having count(*)>=
Linq:
from s in Scores
where s.CNO.StartsWith("")
group s by s.CNO
into cc
where cc.Count() >=
select cc.Average( c => c.DEGREE)
Lambda:
Scores.Where( s => s.CNO.StartsWith("") )
.GroupBy( s => s.CNO )
.Where( cc => ( cc.Count() >= ) )
.Select( cc => cc.Average( c => c.DEGREE) )
Linq: SqlMethod
like也可以这样写:
s.CNO.StartsWith("") or SqlMethods.Like(s.CNO,"%")
.分组
查询Score表中至少有名学生选修的并以开头的课程的平均分数。
select avg(degree) from score where cno like '%' group by Cno having count(*)>=
Linq:
from s in Scores
where s.CNO.StartsWith("")
group s by s.CNO
into cc
where cc.Count() >=
select cc.Average( c => c.DEGREE)
Lambda:
Scores.Where( s => s.CNO.StartsWith("") )
.GroupBy( s => s.CNO )
.Where( cc => ( cc.Count() >= ) )
.Select( cc => cc.Average( c => c.DEGREE) )
Linq: SqlMethod
like也可以这样写:
s.CNO.StartsWith("") or SqlMethods.Like(s.CNO,"%")
. 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()
（