What is difference between select and selectmany in Entity Framework core?

In Entity Framework Core, Select and SelectMany are LINQ methods that serve different purposes when querying data from a database.

1. Select:

   • Select is used for picking specific properties from entities.
   • Example:

   User Class:

   public class User
   {
       public int Id { get; set; }
       public string Name { get; set; }
       public int Age { get; set; }
       public ICollection<Order> Orders { get; set; }
   }
   

   Order Class:

   public class Order
   {
       public int Id { get; set; }
       public string ProductName { get; set; }
       public decimal Price { get; set; }
   }
   

   Select:

   var result = dbContext.Users
                        .Where(u => u.Age > 18)
                        .Select(u => new { Name = u.Name, Age = u.Age });
   

   • In this example, we're selecting only the Name and Age properties from the User entity where the age is over 18.

💡 Note: EF Core Equivalent of Select in SQL is SELECT query.

SELECT Name, Age
FROM Users
WHERE Age > 18;

2. SelectMany:

   • SelectMany combines nested collections into a single collection. It's commonly used when you have a collection containing nested collections and you want to merge them into one.
   • Example:

   User Class:

   public class User
   {
       public int Id { get; set; }
       public string Name { get; set; }
       public int Age { get; set; }
       public ICollection<Order> Orders { get; set; }
   }
   

   Order Class:

   public class Order
   {
       public int Id { get; set; }
       public string ProductName { get; set; }
       public decimal Price { get; set; }
   }
   

   SelectMany:

   var result = dbContext.Users
                        .SelectMany(u => u.Orders);
   

   • In this example, assuming a User entity has a collection navigation property Orders, SelectMany will flatten all the Orders collections from all users into a single collection.

💡 Note: EF Core Equivalent of SelectMany in SQL is by using JOINS in SELECT query.

SELECT u.Name, o.ProductName, o.Price
FROM Users u
JOIN Orders o ON u.Id = o.UserId;

So, in essence:

• Select is used for picking specific properties from entities.
• SelectMany is for flattening nested collections into a single collection.