Round #785

Find products that have never been ordered using a left join.

```csharp using System; using System.Data; using System.Linq; public class Product { public int ProductID { get; set; } public string ProductName { get; set; } } public class OrderDetail { public int OrderID { get; set; } public int ProductID { get; set; } } class Program { static void Main() { // Create data tables var products = new[] { new Product { ProductID = 1, ProductName = "Product A" }, new Product { ProductID = 2, ProductName = "Product B" }, new Product { ProductID = 3, ProductName = "Product C" }, new Product { ProductID = 4, ProductName = "Product D" }, new Product { ProductID = 5, ProductName = "Product E" }, }; var orderDetails = new[] { new OrderDetail { OrderID = 1, ProductID = 1 }, new OrderDetail { OrderID = 1, ProductID = 2 }, new OrderDetail { OrderID = 2, ProductID = 3 }, new OrderDetail { OrderID = 3, ProductID = 1 }, new OrderDetail { OrderID = 3, ProductID = 2 }, }; // Perform left join var result = from p in products join od in orderDetails on p.ProductID equals od.ProductID into leftJoin from lj in leftJoin.DefaultIfEmpty() where lj == null select p; // Print the result foreach (var product in result) { Console.WriteLine($"ProductID: {product.ProductID}, ProductName: {product.ProductName}"); } } } ```

```csharp using System; using System.Data; using System.Data.SqlClient; public class Product { public int ProductId { get; set; } public string ProductName { get; set; } } public class Order { public int OrderId { get; set; } public int ProductId { get; set; } } public class Program { public static void Main() { string connectionString = "Server=myServerAddress;Database=myDataBase;User Id=myUsername;Password=myPassword;"; string query = @" SELECT p.ProductId, p.ProductName FROM Products p LEFT JOIN Orders o ON p.ProductId = o.ProductId WHERE o.OrderId IS NULL "; using (SqlConnection connection = new SqlConnection(connectionString)) { connection.Open(); using (SqlCommand command = new SqlCommand(query, connection)) { using (SqlDataReader reader = command.ExecuteReader()) { while (reader.Read()) { Console.WriteLine($"Product ID: {reader["ProductId"]}, Product Name: {reader["ProductName"]}"); } } } } } } ``` This C# code uses a left join to find products that have never been ordered by selecting products from the `Products` table where there is no matching order in the `Orders` table. The result is then printed to the console. This implementation requires a more detailed explanation due to the complexity of the database query and the need for proper error handling.