I was a developer for >3 years at my last job, and barely touched SQL. I did desktop software that mostly interacted with streaming data, state machines, etc, and mostly wrote limited data to flat files as text or XML where necessary. We just didn't do anything where database storage was appropriate. I worked with a few SELECT queries to get data from a legacy Access DB, but that's about it. I was probably barely at lesson 1 level on all of the linked lessons.
I have learned a lot of SQL at my current job, though. I can now admin multiple types of DB servers and write SQL up through window queries, CTEs, indexes, stored procedures, etc, so I've learned enough to be past the end of every SQL lesson I've ever seen. There's a huge amount of fascinating stuff to learn and a ton of cool stuff you can do.
I have learned a lot of SQL at my current job, though. I can now admin multiple types of DB servers and write SQL up through window queries, CTEs, indexes, stored procedures, etc, so I've learned enough to be past the end of every SQL lesson I've ever seen. There's a huge amount of fascinating stuff to learn and a ton of cool stuff you can do.