What does the N+1 SELECT problem mean when working with a database, and how does it manifest?

The N+1 SELECT problem occurs when, during data loading from a database, initially one query is executed to get a list of main entities (1 SELECT), and then for each of these entities, a separate query is executed to fetch related data (N SELECTs).

For example, loading a list of users and their orders:

1. Query to get all users.
2. For each user, a separate query to get their orders.

This leads to a large number of queries, reducing performance.

The solution is to use eager loading with JOINs or subqueries to fetch all data in one or minimal queries.