Even after the rise of NoSQL databases, relational databases still have a place in this world and form the backbone of many transaction-oriented systems. They may lack the scalability of NoSQL but the advantages they offer in terms of relational design cannot be matched with any NoSQL database.
We generally use MySQL with sequelize ORM in expressjs projects. Express and Sequelize will be introduced in later chapters but you still need solid foundations in SQL to really use them to the fullest.
The best place to get a good grasp on SQL concepts would be to follow the SQL tutorial on https://www.w3schools.com/sql/
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