SQL, the popular programming language used to manage data in a relational database, is used in a ton of apps. Khan Academy's introductory course to SQL will get you started writing SQL in an interactive editor.
The free course takes you from the basics of creating database tables and selecting data to more advanced SQL queries and tasks, such as joining multiple tables together and modifying databases.
The course can be helpful even if you're not a database administrator. As Khan Academy notes on their blog:
It's not just the database admins that are using it; it's the engineers that are writing code to send data to it, it's the product managers that are making decisions based on it, it's the marketing managers checking if their campaign was successful.
It's a useful skill to learn.
Intro to SQL: Querying and managing databases [Khan Academy]