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SQL、LINQ、Lambda 三种用法(转)

SQL、LINQ、Lambda 三种用法
颜色注释: SQL LinqToSql Lambda QA
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) )
Linq: SqlMethod
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 }


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