LINQ

It stands for Language Integrated Query. LINQ is a set of features introduced in Visual Studio 2008 that provides the query facilities into .NET framework language like C#, VB.NET.

Basic LINQ Queries 

public class DALEmployee

    {

       //creating the datacontext instance

       DbConnectionDataContext data = new DbConnectionDataContext();

       //**Retrieve data using LINQ Query**

       public List<tbl_Employee> GetAllEmployees()

       {

           var empdata = from emp in data.tbl_Employees

                         orderby emp.EmployeeID descending

                         select emp;

           return empdata.ToList<tbl_Employee>();

       }

       //**Where Condition in LINQ Query**

       public List<tbl_Employee> GetEmployeebyID(int EmployeeID)

       {

           var empdata = from emp in data.tbl_Employees

                         where emp.EmployeeID == EmployeeID

                         select emp;

           return empdata.ToList();

       }

       //**Insert Data using LINQ Query**

       public int InsertData(tbl_Employee employee)

       {

           data.tbl_Employees.InsertOnSubmit(employee);

           data.SubmitChanges();

           return employee.EmployeeID;

       }

       //**Update Data using LINQ Query**

       public int UpdateData(tbl_Employee employee)

       {

           tbl_Employee objEmployee = data.tbl_Employees.Single(course => course.EmployeeID == employee.EmployeeID);

           objEmployee.Telephone= employee.Telephone;

           objEmployee.Address2 = employee.Address2;

           data.SubmitChanges();

           return employee.EmployeeID;

       }

       //**Delete Data using LINQ Query**

       public bool DeleteData(int employeeID)

       {

           tbl_Employee objEmployee = data.tbl_Employees.Single(course => course.EmployeeID == employeeID);

           data.tbl_Employees.DeleteOnSubmit(objEmployee);

           data.SubmitChanges();

           return true;

       }

       //**Retrieve data using SP**

       public void GetdatausingSP(int EmployeeID)

       {

           var returndata = (from c in data.SP_GetEmployeeData(1)

                             select new tbl_Employee

                             {

                                FirstName  = c.FirstName,

                                LastName=c.LastName

                             }).ToList<tbl_Employee>();

       }

      

    }

References:
http://msdn.microsoft.com/en-us/library/bb397926.aspx

Interview Questions and Answers:
http://aspnet-jitendra.blogspot.sg/2013/07/linq-interview-questions-and-answers.html

http://www.indiabix.com/technical/dotnet/language-integrated-query-linq/