Maninder $ingh I would say that it is important to learn SQL because you need to store the user data somewhere, isn't it?
But since it is something that you haven't started yet then I would like you to have a look at NoSql databases like mongoDB and Cassandra. But knowing how to write SQL queries will surely help you maintain your data.
Every full stack developer should know either of the one.
SQL or NoSQL
Avinesh It depends on the data that you need to store. I have never learnt nosql dbs. But I know SQL and if you need to store data with many relations think that ia still better using a SQL database than a NoSQL.