FreeCodeCamp Great resource to learn to code, also have courses that are specific to SQL (such as the Relational Database course).
Codecademy Other languages are also avaialable to learn on Codecademy. They offer a solid begineer course for free.
W3Schools Amazing place to learn any SQL/General Code related topics.
Coursera If you enjoy more structured class style learning they offer fantastic courses for a monthy or by specific class price for each.
Database Management Systems
PostgreSQL Easy to install and get started, reasonable interface, open source. Completely free.
MySQL Companies may use a version of MySQL which is ran through Oracle. Great choice to use. Very popular open source DBMS and completely free for the community edition.
Microsoft SQL server Companies may use a similar product to this as well . The Express version is free to use for personal data, and is great tool to learn.