The sample SQL statements in coursebooks are usually simple and easy to understand. They even read like English sentences, giving the impression that SQL is rather simple and easy to learn.
Actually, such a SQL statement consisting of only a few lines of code can only be found in coursebooks and training courses. In real-world businesses, the amount of SQL code is measured by KB instead of the number of lines. One SQL statement having several hundred lines of code and N layers of nested subqueries often reaches 3KB to 5KB in size. Such SQL statements are not easy to learn at all but rather a nightmare even to professional programmers.