MongoDB vs SQL: Day 3-5` presents a comparison of using MongoDB versus SQL for storing and managing data in an application, specifically focusing on adding phone numbers to a contact structure. The author highlights the differences between the two approaches, including the complexity of relational database management systems (RDBMS) like SQL and their limitations, such as dealing with "zombies" - entities that don't have phones but are still included in the data. In contrast, MongoDB allows for native storage of a list of phone numbers without requiring additional tables or joins, significantly reducing the time and effort required to manage this data. The comparison demonstrates how MongoDB's flexible document-based data model can simplify data management and reduce complexity compared to traditional RDBMS approaches.