SQL stands for Structured Query Language. It is a special purpose standard programming language. SQL is used to issue instructions relating to the addition, retrieval, removal and access of data within a relational database management system. Simply put, it’s a language you can use to access data that’s stored within a database.
MySQL is an open source relational database management system (RDBMS). It is a software that’s used to store and manipulate data. Open source means “it is free to use and undergoes frequent changes to enhance security and user friendliness.” MySQL is an RDBS that is popularly used on the web as opposed to offline data management.
SQL is a query language as it is a 5th generation language it's more closed to human language can understand it easily and mySQL is a programme or OS which can understand and manipulate the SQL Queries.
If you know SQL, so how you don't know about MySQL?
Write here that you don't know about MSSQL, PostgreSQL etc.
This is a really analogical with car, but:
I know how drive a car, but i don't know what is a car.