SQL means "Structured Query Language", and is used to *search* through 
existing relational database management systems (RDBMS). 

In 2020, the most used RDBMS options[1] are (most-lowest):
1. Oracle DB
2. MySQL 
3. Microsoft SQL Server
4. PostgreSQL
5. SQLite

Google trends (2020) ranked the same systems in the order:
1. MySQL
2. Microsoft SQL Server 
3. PostgreSQL 
4. Oracle DB
5. SQLite 

MySQL, PostgreSQL and SQLite are Open Source and 'Free'.

- MySQL's shell handles JavaScript, Python and formerly Perl. 
  You can find drivers for the mysql connectors in most languages.
 
- PostgreSQL's interface layer is open to most languages. The 
  psql shell interpets PL/pgSQL, a custom language for the DBMS. 

- SQLite has a single custom shell language, but supports bindings
  in most well known programming languages. SQLite is installed 
  automatically on some Unix/Linux distributions. 

References: 
[1] https://db-engines.com/en/ranking