Round #662

Use ROW_NUMBER to deduplicate orders, keeping the most recent per customer.

To deduplicate orders while keeping the most recent one per customer using `ROW_NUMBER()` in SQL Server, you can follow these steps: 1. Use a Common Table Expression (CTE) or subquery to assign a row number to each order for each customer based on the order date. 2. Filter out the rows where the row number is greater than 1. Here's an example of how you can achieve this: ```sql WITH RankedOrders AS ( SELECT OrderID, CustomerID, OrderDate, ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate DESC) AS RowNum FROM Orders ) SELECT OrderID, CustomerID, OrderDate FROM RankedOrders WHERE RowNum = 1; ``` ### Explanation: - **CTE (`RankedOrders`)**: - `ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate DESC) AS RowNum`: This assigns a unique row number to each order for each customer, ordered by the `OrderDate` in descending order. The most recent order will have a row number of 1. - **Final SELECT**: - We select only those rows where `RowNum = 1`, which means we keep the most recent order for each customer. ### Example Data: Assume you have an `Orders` table with the following data: | OrderID | CustomerID | OrderDate | |---------|------------|-----------------| | 101 | CUST001 | 2023-04-01 | | 102 | CUST001 | 2023-05-01 | | 103 | CUST002 | 2023-06-01 | | 104 | CUST002 | 2023-07-01 | | 105 | CUST001 | 2023-08-01 | ### Result: The query will return: | OrderID | CustomerID | OrderDate | |---------|------------|-----------------| | 102 | CUST001 | 2023-05-01 | | 104 | CUST002 | 2023-07-01 | This result keeps the most recent order for each customer.

To implement the request of using `ROW_NUMBER()` to deduplicate orders while keeping the most recent one per customer, we can utilize SQL Server's window function directly in a query. This approach avoids the need for additional libraries or complex logic in C#. Here is the complete implementation: ```csharp using Microsoft.EntityFrameworkCore; using System.Linq; public class Order { public int Id { get; set; } public int CustomerId { get; set; } public DateTime OrderDate { get; set; } // Other properties... } public class ApplicationDbContext : DbContext { public DbSet<Order> Orders { get; set; } protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) { optionsBuilder.UseSqlServer("YourConnectionStringHere"); } } public class OrderService { private readonly ApplicationDbContext _context; public OrderService(ApplicationDbContext context) { _context = context; } public List<Order> GetMostRecentOrdersPerCustomer() { return _context.Orders .FromSqlRaw( "SELECT * FROM (SELECT *, ROW_NUMBER() OVER(PARTITION BY CustomerId ORDER BY OrderDate DESC) AS rn FROM Orders) WHERE rn = 1") .ToList(); } } ``` ### Explanation: 1. **Order Entity**: This represents the order entity with properties such as `Id`, `CustomerId`, and `OrderDate`. 2. **ApplicationDbContext**: This is the Entity Framework Core context that includes a `DbSet<Order>`. 3. **OrderService**: This service class contains a method `GetMostRecentOrdersPerCustomer` which uses SQL Server's `ROW_NUMBER()` window function to deduplicate orders by customer, keeping only the most recent one. - The `FromSqlRaw` method is used to execute a raw SQL query that includes the `ROW_NUMBER()` window function. - The subquery partitions the rows by `CustomerId` and assigns a row number based on the `OrderDate` in descending order. - The outer query filters for rows where the row number (`rn`) is 1, effectively keeping only the most recent order per customer. This approach leverages SQL Server's powerful window functions directly within the database, ensuring efficient and performant deduplication.