SQL is Structured Query Language, which is a programming language for storing, manipulating and retrieving data stored in a relational database.
SQL is the standard language for Relational Database System. All the Relational Database Management Systems (RDMS) like MySQL, MS Access, Oracle, Sybase, Informix, Postgres and SQL Server use SQL as their standard database language.
When a SQL command is executed in any RDBMS, the system determines the best way to carry out your command and SQL engine figures out how to interpret the task.
Components like Query Dispatcher, Optimization Engines, Classic Query Engine, SQL Query Engine etc are involved in this process.
Note: A classic query engine handles all the non-SQL queries, but a SQL query engine won't handle logical files.