Explain what the INCLUDE operator in SQL is and how it is used in queries.

The SQL operator INCLUDE is not directly available, but in the context of ORM, such as Entity Framework in C#, Include is a method for eager loading of related entities.

It is used to load related data along with the main entity during selection, preventing the "N+1 query" problem.

Example of usage in Entity Framework:

var orders = context.Orders
    .Include(o => o.Customer)  // load related Customer data along with Orders
    .ToList();

Here, Include indicates that when selecting orders, the information about customers related to these orders should be loaded immediately.

Thus, Include helps optimize queries and avoid additional database calls when working with navigation properties.