SQL、LINQ、Lambda 三种用法(转)
1、 查询Student表中的所有记录的Sname、Ssex和Class列。
select sname,ssex,class from studentLinq: 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 teacherLinq: from t in Teachers.Distinct() select t.DEPARTLambda: Teachers.Distinct().Select( t => t.DEPART)
3、 查询Student表的所有记录。
select * from studentLinq: from s in Students select sLambda: 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 80Linq: from s in Scores where s.DEGREE >= 60 && s.DEGREE < 80 select sLambda: 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 sLambda: 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 sLambda: 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 sLambda: 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 DESCLinq: from s in Students orderby s.CLASS descending select sLambda: 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 DESCLinq:(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 sLambda: 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 "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(*)>=5Linq: 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: SqlMethodlike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) < 90Linq: 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.snoLinq: 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.cnoLinq: 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.cnoLinq: 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 }