An excellent introductory resource is “w3.schools.com” which has interactive exercises for you to practice in real time. Once you have mastered those, try “sqlzoo.net” ** which has more difficult questions – take your time, take notes. Use your “Exploring IT” textbook for some extra help. With patience you will get the hang of it.
Also attached find a SQL summary cheat v4 that is very useful but it needs some hard study time. This SQL stuff does not just jump into your head like magic.
** PK stand for Primary Key and FK stands for Foreign Key.