All dynamic website and web applications are using relational database for saving the data. These days many websites are also using Big Data store.
For a developer knowledge of SQL is very important as it is required write SQL query to perform database operations.
For example you developed a form for user registration then you would like to save data into back end database. And for this you have to write SQL query and run it against database to store data.
So its very import for a developer, database admin and anyone involved in database access function should learn SQL.
You can learn SQL from following tutorials: