LINQ - Joins in C#
Example: 
using System;
using System.Linq;
using System.Xml.Linq;

namespace ForgetCode
{   
    public class MainClass
    {
        public static void Main()
        {
            dat customers = new dat(@"c:\data.mdf");
            dat2 distributors = new dat2(@"c:\data.mdf");

            var innerJoinQuery =
            from cust in customers
            join dist in distributors on cust.City equals dist.City
            select new { CustomerName = cust.Name, DistributorName = dist.Name };
            
        }
    }
}