SQL fundamentals are those concepts and topics requiring a firm understanding to solve real-world problems. The exact order in which new SQL users should learn the fundamentals is debatable; however, I believe most experienced users would agree starting with THE BIG SIX and then moving onto JOINS provides a solid foundation to build from. Below is the order in which I learned the fundamentals several years ago. It is deceptively short given many of the topics can be further broken down into subtopics (e.g., the big six contains six major clauses and there are several types of joins).
THE BIG SIX
JOINS
UNION & UNION ALL
CASE EXPRESSIONS
FUNCTIONS
SUBQUERIES
COMMON TABLE EXPRESSIONS
RECURSION
VIEWS
Keep in mind, my learning path was driven by the problems I was required to solve. Depending on the problems you are faced with, your path may be different. As you finish each topic or concept, I strongly encourage you to practice what you have learned by solving practice problems. By doing so, you are confirming and solidifying your understanding. I’ve provided many practice problems here. The problems cover the fundamentals and vary in difficulty from easy to hard.