How to use lambda expression in C#?

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I want to sql query convert to lambda expression.How to use lambda expression this query.

Class table:
Id,Name,Surname

School table:
Id,Name,SchoolInformation

Class Table:

1 Alex   John  
1 Alex   Michael
1 Alex   Martin
2 Mary   Kelvin

School Table:

1 Alex
2 Mary
3 Justin

What I have tried:

SQL

Select distinct c.id as id,c.Name from class as c school as s where c.id=s.id order by 1

How to convert to lambda expression this query.
I want to result:

1 Alex
2 Mary

Posted 5-Oct-19 11:25am

Member 14169626

Updated 7-Oct-19 0:16am

v3

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Mohibur Rashid 5-Oct-19 19:23pm

Do you know what is lambda expression?
How would you do if you do not use lambda expression?

You query is incorrect
Select distinct
t.id as id ------- WHERE DO YOU GET YOUR T?
, t.Name
from
class as c
school as s
where
c.id=s.id
order by 1

Member 14169626 6-Oct-19 2:38am

sorry question is uptated c. lambda expression x=>x.id like to write the query.
But I don't know the distinct and combination of the two tables

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Maciej Los · Answer 1 · 2019-10-07T00:16:00

Sorry, your question is unclear and/or your database is wrongly designed, because schoold.id and class.id can NOT be equal! A database model should looks like:

1 School => ∞ YearBooks
1 YearBook => ∞ Classes
1 Class => ∞ Students

So, you need at least 4 objects (tables):
1. Yearbooks table - describes a school year
2. Classes table - contains all classes, each entry describes a class for a particular school year
3. Students table - contains all registered students, each entry describes a particular student
4. StudentsInClass table - (many to many relationship between students and classes) contains ids of class and student

BTW: I'd recommend to replace old-fashioned SQL programming

SQL

Select distinct c.id as id,c.Name
from class as c, school as s
where c.id=s.id
order by 1

to new one (using Join's[^]):

SQL

SELECT DISTINCT c.id as id, c.Name
FROM class as c
    INNER JOIN school as s ON c.id=s.id
ORDER BY c.id

For further details, please see: Visual Representation of SQL Joins[^]

When you finish changing a database model and queries into proper ones, i'd recommend to to study this site: http://linq101.nilzorblog.com/linq101-lambda.php[^]
It might be helpfull in understang how to write Linq queries using lambda expressions.

Rajat-Indiandotnet · Answer 2 · 2019-10-05T15:59:00

I hope the following way helps you.

List<Student> _lstStudent = new List<Student>();
List<School> _lstSchool = new List<School>();
_lstSchool.Add(new School() { Id = 1, Name = "MVM" });
_lstSchool.Add(new School() { Id = 2, Name = "DPS" });
_lstSchool.Add(new School() { Id = 3, Name = "JHK" });

_lstStudent.Add(new Student() { Id = 1, SchoolID = 1, Name = "Rajat", Surname = "Jaiswal" });
_lstStudent.Add(new Student() { Id = 2, SchoolID = 1, Name = "Aayush", Surname = "Jaiswal" });
_lstStudent.Add(new Student() { Id = 3, SchoolID = 2, Name = "Aman", Surname = "Jaiswal" });
_lstStudent.Add(new Student() { Id = 4, SchoolID = 3, Name = "Bittu", Surname = "Jaiswal" });



var tmp = from aStudent in _lstStudent
          join aSchool in _lstSchool on aStudent.SchoolID equals aSchool.Id
          select new
          {
              aStudent.Id,
              aStudent.Name,
              aStudent.Surname,
              SchoolName = aSchool.Name
          };

var tmp1 = _lstStudent.Join(_lstSchool, aStudent => aStudent.SchoolID,
                                        aSchool => aSchool.Id,
                                        (aStudent, aSchool) => new
                                        {
                                            aStudent.Id,
                                            aStudent.Name,
                                            aStudent.Surname,
                                            SchoolName = aSchool.Name
                                        });

foreach (var a in tmp)
{
    Console.WriteLine("Student Name {0} and Surname  {1}  Study in {2} school",a.Name, a.Surname,a.SchoolName);

}

Console.ReadKey();

How to use lambda expression in C#?

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