SQL is standard language to access and manipulation data in MySQL and Oracle.
Although SQL is quite easy to learn and has limited syntax, but the concept of using mainly DDL (Data Definition Language) ,DML (Data Manipulation Language) and DCL (Data Control Language) take time to understand or to get accustomed to make use of when it is required. (ref)
As far as , Learning SQL and grasping the syntax may be quite easy to begin with (lot of online resource are there) , but to get the required data for the real world problem might be not that much easy and may require practice.
SQLZOO is website where you have opportunity to practice these problem, to sharpen your skill and to test what you have acquired. The best thing of this website , it support all type of database engine (Oracle, MySQL etc) and has many problem to practice.
Problem start with a scenario and will ask you to use specific DML , to get the required results.
Great website to start with your SQL training and hone your ability.