If you know SQL, is learning NoSQL/ MySQL easy?

MySQL is nothing but an instance of SQL language. So if you know SQL, its as good as telling you know MySQL except for the few specific syntax which you might need to learn.

NoSQL on the other hand refers to mix of structured and non-structured data stored in big data systems. Based on the storage model it uses, the query construct might be different and will not be 100% similar to SQL. So it may be a good learning curve for a person coming from SQL background