Round #576

Find duplicate rows in an emails table based on email address.

```csharp using System; using System.Data; using System.Linq; class Program { static void Main() { // Create a sample DataTable DataTable emails = new DataTable(); emails.Columns.Add("email", typeof(string)); emails.Columns.Add("name", typeof(string)); emails.Rows.Add("test@gmail.com", "Test"); emails.Rows.Add("test2@gmail.com", "Test2"); emails.Rows.Add("test@gmail.com", "Test"); emails.Rows.Add("test3@gmail.com", "Test3"); emails.Rows.Add("test2@gmail.com", "Test2"); // Find duplicate rows var duplicateRows = emails.AsEnumerable() .GroupBy(row => row.Field<string>("email")) .Where(g => g.Count() > 1) .SelectMany(g => g); // Print the duplicate rows foreach (var row in duplicateRows) { Console.WriteLine($"{row["email"]}\t{row["name"]}"); } } } ```

```csharp using Microsoft.EntityFrameworkCore; using System; using System.Collections.Generic; using System.Linq; public class Email { public int Id { get; set; } public string EmailAddress { get; set; } } public class EmailContext : DbContext { public DbSet<Email> Emails { get; set; } protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) { optionsBuilder.UseSqlServer(@"Data Source=(localdb)\mssqllocaldb;Initial Catalog=EmailsDB;Integrated Security=True"); } } public class DuplicateEmailFinder { private readonly EmailContext _context; public DuplicateEmailFinder(EmailContext context) { _context = context; } public List<Email> FindDuplicateEmails() { return _context.Emails .GroupBy(e => e.EmailAddress) .Where(g => g.Count() > 1) .SelectMany(g => g) .ToList(); } } class Program { static void Main(string[] args) { using var context = new EmailContext(); context.Database.EnsureCreated(); var finder = new DuplicateEmailFinder(context); var duplicateEmails = finder.FindDuplicateEmails(); foreach (var email in duplicateEmails) { Console.WriteLine($"Duplicate email found: {email.EmailAddress}"); } } } ``` This C# code uses Entity Framework Core to connect to a SQL Server database and find duplicate rows in the `Emails` table based on the `EmailAddress` column. Here's how it works: 1. We define an `Email` class to represent the `Emails` table. 2. We create an `EmailContext` class that inherits from `DbContext` and contains a `DbSet<Email>` to represent the `Emails` table. 3. We override the `OnConfiguring` method to specify the database connection string. 4. We create a `DuplicateEmailFinder` class that takes an instance of `EmailContext` in its constructor. 5. The `FindDuplicateEmails` method uses LINQ to group the emails by their addresses, filter the groups to only include those with more than one email, and then select all emails from these groups. 6. In the `Main` method, we create an instance of `EmailContext`, ensure the database is created, and then use the `DuplicateEmailFinder` to find and print all duplicate emails. Note: This code assumes that the `Emails` table has an `Id` column and an `EmailAddress` column. You may need to adjust the code to match your actual table schema.