sql 、linq、lambda 查询语句的区别
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
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1. 查询Student表的所有记录。
2. select * from student
3. Linq:
4. from s in Students
5. select s
6. Lambda:
7. Students.Select( s => s)
2 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
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1. Linq:
2. from s in Students
3. select new {
4. s.SNAME,
5. s.SSEX,
6. s.CLASS
7. }
8. Lambda:
9. Students.Select( s => new {
10. SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
11. })
3.distinct 去掉重复的
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1. 查询教师所有的单位即不重复的Depart列。
2. select distinct depart from teacher
3. Linq:
4. from t in Teachers.Distinct()
5. select t.DEPART
6. Lambda:
7. Teachers.Distinct().Select( t => t.DEPART)
4.连接查询 between and
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1. 查询Score表中成绩在60到80之间的所有记录。
2. select * from score where degree between 60 and 80
3. Linq:
4. from s in Scores
5. where s.DEGREE >= 60 && s.DEGREE < 80
6. select s
7. Lambda:
8. Scores.Where(
9. s => (
10. s.DEGREE >= 60 && s.DEGREE < 80
11. )
12. )
5.在范围内筛选 In
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1. select * from score where degree in (85,86,88)
2. Linq:
3. from s in Scores
4. where (
5. new decimal[]{85,86,88}
6. ).Contains(s.DEGREE)
7. select s
8. Lambda:
9. Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE))
6.or 条件过滤
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1. 查询Student表中"95031"班或性别为"女"的同学记录。
2. select * from student where class ='95031' or ssex= N'女'
3. Linq:
4. from s in Students
5. where s.CLASS == "95031"
6. || s.CLASS == "女"
7. select s
8. Lambda:
9. Students.Where(s => ( s.CLASS == "95031" || s.CLASS == "女"))
7.排序
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1. 以Class降序查询Student表的所有记录。
2. select * from student order by Class DESC
3. Linq:
4. from s in Students
5. orderby s.CLASS descending
6. select s
7. Lambda:
8. Students.OrderByDescending(s => s.CLASS)
8.count（）行数查询
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1. select count(*) from student where class = '95031'
2. Linq:
3. ( from s in Students
4. where s.CLASS == "95031"
5. select s
6. ).Count()
7. Lambda:
8. Students.Where( s => s.CLASS == "95031" )
9. .Select( s => s)
10. .Count()
10.avg()平均
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1. 查询'3-105'号课程的平均分。
2. select avg(degree) from score where cno = '3-105'
3. Linq:
4. (
5. from s in Scores
6. where s.CNO == "3-105"
7. select s.DEGREE
8. ).Average()
9. Lambda:
10. Scores.Where( s => s.CNO == "3-105")
11. .Select( s => s.DEGREE)
11.子查询
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1. 查询Score表中的最高分的学生学号和课程号。
2. select distinct s.Sno,c.Cno from student as s,course as c ,score as sc
3. where s.sno=(select sno from score where degree = (select max(degree) from score))
4. and c.cno = (select cno from score where degree = (select max(degree) from score))
5. Linq:
6. (
7. from s in Students
8. from c in Courses
9. from sc in Scores
10. let maxDegree = (from sss in Scores
11. select sss.DEGREE
12. ).Max()
13. let sno = (from ss in Scores
14. where ss.DEGREE == maxDegree
15. select ss.SNO).Single().ToString()
16. let cno = (from ssss in Scores
17. where ssss.DEGREE == maxDegree
18. select ssss.CNO).Single().ToString()
19. where s.SNO == sno && c.CNO == cno
20. select new {
21. s.SNO,
22. c.CNO
23. }
24. ).Distinct()
12.分组过滤
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1. 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。
2. select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3. Linq:
4. from s in Scores
5. where s.CNO.StartsWith("3")
6. group s by s.CNO
7. into cc
8. where cc.Count() >= 5
9. select cc.Average( c => c.DEGREE)
10. Lambda:
11. Scores.Where( s => s.CNO.StartsWith("3") )
12. .GroupBy( s => s.CNO )
13. .Where( cc => ( cc.Count() >= 5) )
14. .Select( cc => cc.Average( c => c.DEGREE) )
15. Linq: SqlMethod
16. like也可以这样写:
17. s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")
13.分组
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1. 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。
2. select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
3. Linq:
4. from s in Scores
5. where s.CNO.StartsWith("3")
6. group s by s.CNO
7. into cc
8. where cc.Count() >= 5
9. select cc.Average( c => c.DEGREE)
10. Lambda:
11. Scores.Where( s => s.CNO.StartsWith("3") )
12. .GroupBy( s => s.CNO )
13. .Where( cc => ( cc.Count() >= 5) )
14. .Select( cc => cc.Average( c => c.DEGREE) )
15. Linq: SqlMethod
16. like也可以这样写:
17. s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")
14. 多表查询
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1. Linq:
2. from c in Courses
3. join sc in Scores
4. on c.CNO equals sc.CNO
5. select new
6. {
7. sc.SNO,c.CNAME,sc.DEGREE
8. }
9. Lambda:
10. Courses.Join ( Scores, c => c.CNO,
11. sc => sc.CNO,
12. (c, sc) => new
13. {
14. SNO = sc.SNO,
15. CNAME = c.CNAME,
16. DEGREE = sc.DEGREE
17. })
18. .Average()