A misconception that I held for a really long time was that systems that use SQL (Structured Query Language) are by definition relational databases, and systems that don’t (e.g., MongoDB, Cassandra) are non-relational. It’s easy to understand whether a database uses SQL or not, but what does it mean when we say a database is relational? At a minimum, relational databases expose data in the form of “relations,” another word for tables with rows and columns.