Reddit reviews Joe Celko's SQL for Smarties: Advanced SQL Programming (The Morgan Kaufmann Series in Data Management Systems)

What I'm talking about is pretty basic knowledge. I'd suggest googling for it first and reading through that.

If you're using an IDE (seriously, use an IDE), look for debugging in <your IDE>. It should give you what you need. You should be able to stop execution at a given breakpoint, step into and over method calls, and inspect the values of object and expressions. Bonus points for learning how to "Run to the cursor position" instead of dropping breakpoints everywhere and for figuring out conditional breakpoints. It's more complicated, but being able to debug a remote application is also really useful for most web applications.

After that, if you want to get into more systematic debugging, I recommend looking at Debugging: The 9 Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems

For SQL, again, read the basics and then play around with it in an SQL application. Honestly, for learning purposes, if you have it, MS Access is pretty good. MySQL is probably the most accessible free tool.

Learn how to get data, filter it, order it from a single table. Then how to use grouping and group level filtering with GROUP BY and HAVING. Then learn inner and outer joins.

That's going to put you way in front of most young devs.

If you want to really get into it, I recommend Joe Celko's SQL for Smarties, but that's kind of overkill.

My personal favorite is SQL for Smarties by Celko:
https://www.amazon.com/dp/0128007613/ref=cm_sw_r_cp_apa_1wTRAbG6R88WS

One warning - he doesn't hold back on advanced SQL, so you'll run into a handful of cases where MySQL doesn't support what you're trying to do, or even worse, does the wrong thing. You should be fine in postgres.

Check out The Accidental DBA from Redgate. It covers the basics from a standpoint of someone who has no background but is now responsible for keeping a SQL server running. Scan through it and see if there is anything you don't know. if so then dive into the "Inside Out" series of books from Microsoft Press.

if you want to get really into the weeds find SQL for Smarties for some in-depth discussion on why SQL (the language) behaves the way it does.

https://smile.amazon.com/Joe-Celkos-SQL-Smarties-Fifth/dp/0128007613/

SQL for Smarties by Joe Celko