Get started on mastering the one language binding the entire database industry. That language is SQL, and how it works is must-have knowledge for anyone involved with relational databases, and surprisingly also for anyone involved with NoSQL databases. SQL is universally used in querying and reporting on large data sets in order to generate knowledge to drive business decisions.
Good knowledge of SQL is crucial to anyone working with databases, because it is with SQL that you retrieve data, manipulate data, and generate business results. Every relational database supports SQL for its expressiveness in writing queries underlying reports and business intelligence dashboards. Knowing how to write good queries is the foundation for all work done in SQL, and it is a foundation that Clare Churcher's book, Beginning SQL Queries, 2nd Edition, lays well.