Pareto Principle, also known as the 80/20 rule, states that 80% of the output comes from 20% of the input. How are we going to apply this to MySQL?
If the MySQL documentation is 1,000 pages long (I'm giving an arbitrary number here), then according to the 80/20 rule, in order to learn 80% of MySQL, you just have to study 20% of the documentation which is 200 pages.
But! 200 pages is still a lot. Non-fiction books are almost 200 pages long. What if we apply 80/20 rule to this situation again? So, to learn 80% of the 200 pages, you just need to learn 20% of it which would be 40 pages.
Think about it, in a daily DBA work, do you really need to know 40 pages worth of MySQL documentation? Most likely not, you may have intuitively applied the 80/20 rule again and again until you are left with probably 3 pages worth of MySQL documentation which are the bare essentials to your work.