发布：2023/12/7 15:22:31作者：大数据 来源：大数据 浏览次数：414

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

linq与lambda表达式select in的写法在下面
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()

--自己的例子

IList<DCKeyword> mDCKeywordJoinLst = mDCKeywordLst.Join(
mDCKeywordImportants,
word => word.keyword_id,
impWord => impWord.keyword_id,
(word, impWord) => word
).ToList(); //重点关键词

ef core contains数组，查询包含数组的contains多个字符串contains多个关键字

var q=_dbcontext.News.AsQueryable();

var keywordsList=keywords.Split(',');//张三,李四

q = q.Where(t => keywordsList.Any(k=>t.NewsContent.Contains(k)));