SQL, LINQ and Lambda expressions

First of all, let’s talk about these three completely different things. SQL is the abbreviation of Structured Query Language (Structured Query Language), which everyone is familiar with. The following mainly introduces LINQ. and the basic concepts of Lambda expressions and different implementations of the same query.

Brief introduction

LINQ (Language Integrate Query) is a language integrated query. It establishes a corresponding relationship between objects and data. You can query data collections by accessing memory objects. LINQ query is a language construct in C#. Therefore, developers can nest query expressions similar to SQL statements in C# code to implement data query functions. LINQ is not simply a nested query expression in C#, but uses query expressions as a syntax of C#.
In the .NET class library, LINQ-related class libraries are under the System.Linq namespace. This namespace provides classes and interfaces that support querying using LINQ. The most important of them are the following two classes and two interfaces.
※IEnumerable interface: It represents a data collection that can be queried. A query usually filters the elements in the collection one by one and returns a new IEnumerable interface to save the query results.
※IQueryable interface: It inherits the IEnumerable interface and represents an expression directory tree that can be queried.
※Enumerable class: It implements LINQ standard query operators by providing extension methods for IEnumerable. Including operations such as passing, navigation, sorting, query, join, sum, maximum and minimum.
※Queryable class: It implements LINQ standard query operators by providing extension methods for IQueryable. Including operations such as passing, navigation, sorting, query, join, sum, maximum and minimum.
Lambda expression is actually an anonymous function, which can be said to be a supplement to LINQ. Since there is a correspondence between LINQ query keywords and methods of the IEnumerable interface, there are few query functions that can be used in LINQ query expressions. In actual development, method calls are made through query results or data sources to perform more query operations. Since Lambda expression is an anonymous function, it can be assigned to a delegate, and many of the methods of the IEnumerable interface use function delegation to implement custom operations, conditions and other operations, so Lambda expressions are widely used in LINQ.

Comparison implementation

※Query all contents

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)

※Query by column

select sname,ssex,class from student 
3 Linq: 
4     from s in Students 
5     select new { 
6         s.SNAME, 
7         s.SSEX, 
8         s.CLASS
 9     }
 10 Lambda:
 11     Students.Select( s => new {
 12         SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
 13     })

※distinct deduplication query

 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.DEPART6 Lambda:
7     Teachers.Distinct().Select( t => t.DEPART)

※Two Query within an interval

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     )

※Query within a range

 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))

※Or relational query

 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"))

※Sort

 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)

※Query by number of rows

 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()

※Average query

 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)

※Absent query

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()

※Group

 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")

※Group filtering

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")

※Multiple table joint query

 select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno
 2 Linq: 
 3     from c in Courses 
 4     join sc in Scores 
 5     on c.CNO equals sc.CNO
 6     select new 
 7     { 
 8         sc.SNO,c.CNAME,sc.DEGREE
 9     }
 10 Lambda:
 11     Courses.Join ( Scores, c => c.CNO,
 12                              sc => sc.CNO,
 13                              (c, sc) => new
 14                                         {
 15                                             SNO = sc.SNO,
 16                                             CNAME = c.CNAME,
 17                                             DEGREE = sc.DEGREE
 18                                         })
 19                 .Average()

The above content is some compilation and summary made by me after consulting the information. If there are any shortcomings, please criticize and correct me!

First of all, let’s talk about these three completely different things. SQL is the abbreviation of Structured Query Language (Structured Query Language), which everyone is familiar with. The following mainly introduces the basics of LINQ and Lambda expressions. concepts and different implementations of the same query.

简单介绍

LINQ（Language Integrate Query）是语言集成查询他在对象和数据之间建立一种对应的关系，可以使用访问内存对象的方式查询数据集合。LINQ查询是C#中的一种语言构造。因此开发人员可以再C#代码汇总嵌套类似于SQL语句的查询表达式，从而实现数据查询的功能。LINQ也不是简单地作为C#中嵌套查询表达式，而是将查询表达式作为C#的一种语法。
在.NET类库中，LINQ相关类库都在System.Linq命名空间下，该命名空间提供支持使用LINQ进行查询的类和接口，其中最主要的是以下两个类和两个接口。
※IEnumerable接口：它表示可以查询的数据集合，一个查询通常是逐个对集合中的元素进行筛选操作，返回一个新的IEnumerable接口，用来保存查询结果。
※IQueryable接口：他继承IEnumerable接口，表示一个可以查询的表达式目录树。
※Enumerable类：它通过对IEnumerable提供扩展方法，实现LINQ标准查询运算符。包括过路、导航、排序、查询、联接、求和、求最大值、求最小值等操作。
※Queryable类：它通过对IQueryable提供扩展方法，实现LINQ标准查询运算符。包括过路、导航、排序、查询、联接、求和、求最大值、求最小值等操作。
Lambda表达式实际上是一个匿名函数，它可以说是对LINQ的补充。由于LINQ查询关键字和IEnumerable接口的方法之间有一个对应关系，但是LINQ查询表达式中可以使用的查询功能很少。在实际开发中通过查询结果或数据源进行方法调用，从而进行更多的查询操作。由于Lambda表达式是匿名函数，它可以赋值到一个委托，而在IEnumerable接口的方法中很多通过函数委托来实现自定义运算、条件等操作，所以Lambda表达式在LINQ中被广泛使用。

对比实现

※查询全部内容

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)

※按列查询

select sname,ssex,class from student 
3 Linq: 
4     from s in Students 
5     select new { 
6         s.SNAME, 
7         s.SSEX, 
8         s.CLASS
 9     }
 10 Lambda:
 11     Students.Select( s => new {
 12         SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
 13     })

※distinct去重查询

 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)

※两个区间内查询

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     )

※在一个范围内查询

 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))

※或关系查询

 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"))

※排序

 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)

※行数查询

 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()

※平均值查询

 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)

※潜逃查询

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()

※分组

 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")

※分组过滤

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")

※多表联合查询

 select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno
 2 Linq: 
 3     from c in Courses 
 4     join sc in Scores 
 5     on c.CNO equals sc.CNO
 6     select new 
 7     { 
 8         sc.SNO,c.CNAME,sc.DEGREE
 9     }
 10 Lambda:
 11     Courses.Join ( Scores, c => c.CNO,
 12                              sc => sc.CNO,
 13                              (c, sc) => new
 14                                         {
 15                                             SNO = sc.SNO,
 16                                             CNAME = c.CNAME,
 17                                             DEGREE = sc.DEGREE
 18                                         })
 19                 .Average()

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