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 Studentsselect 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 Studentsselect 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 Scoreswhere s.DEGREE >= 60 && s.DEGREE < 80select 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:
Infrom s in Scoreswhere (new decimal[]{85,86,88}).Contains(s.DEGREE)select s
Lambda:Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE))
Not infrom s in Scoreswhere !(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Anyfrom e in CustomerDemographicswhere !e.CustomerCustomerDemos.Any()select efrom c in Categorieswhere !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 Studentswhere 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 Studentsorderby s.CLASS descendingselect 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 Scoresorderby s.DEGREE descendingorderby 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 Studentswhere 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 Studentsfrom c in Coursesfrom sc in Scoreslet maxDegree = (from sss in Scoresselect sss.DEGREE).Max()let sno = (from ss in Scoreswhere ss.DEGREE == maxDegreeselect ss.SNO).Single().ToString()let cno = (from ssss in Scoreswhere ssss.DEGREE == maxDegreeselect ssss.CNO).Single().ToString()where s.SNO == sno && c.CNO == cnoselect 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 Scoreswhere ss.DEGREE == maxDegreeselect 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 Scoreswhere ss.DEGREE == maxDegreeselect 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 Scoreswhere 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 Scoreswhere s.CNO.StartsWith("3")group s by s.CNOinto ccwhere cc.Count() >= 5select 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 Scoresgroup s by s.SNOinto sswhere ss.Min(cc => cc.DEGREE) > 70 && ss.Max( cc => cc.DEGREE) < 90select 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 Studentsjoin sc in Scoreson s.SNO equals sc.SNOselect 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 Coursesjoin sc in Scoreson c.CNO equals sc.CNOselect 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 Studentsfrom c in Coursesfrom sc in Scoreswhere s.SNO == sc.SNO && c.CNO == sc.CNOselect new { s.SNAME,c.CNAME,sc.DEGREE }

转载于:https://www.cnblogs.com/keepee/p/6120991.html

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