One thing you have to realize is that once you get a little advanced, you have to get to the details of the single SQL implementations, it's not about SQL but about Postgres.
I've found these books really valuable
# SQL Performance Explained Everything Developers Need to Know about SQL Performance
This book fundamentally talks about how to effectively use and leverage the SQL indices. Talks about all the important implementations (Postgres, MySQL, Oracle, SQL Server).
# Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems
This book gets mentioned a bunch around here and for a good reason. There aren't too many concrete resources on making your systems "webscale" and this one is really good.
# PostgreSQL 9.0 High Performance
Discusses all the different settings and tweaks you can do in Postgres. It's crazy how much of a perf gain you can get just by twiddling the parameters of the database, i.e. all the tricks you can do when the single instances are bottle necks.
There's a similar book for MySQL https://www.amazon.com/High-Performance-MySQL-Optimization-R...
# PostgreSQL 9 High Availability Cookbook
Discusses how do you go from 1 Postgres instance to 1+ instance. Talks about replication, monitoring, cluster management, avoiding downtime etc i.e. all the tricks you can do to manage multiple instances. Again there's a similar book for MySQL https://www.amazon.com/MySQL-High-Availability-Building-Cent...
Last but not least check out the postgres documentation, people consider it a standard of what good documentation looks like https://www.postgresql.org/docs/9.6/static/index.html
Also last but not least, read up on relational algebra (the foundation of SQL) https://en.wikipedia.org/wiki/Relational_algebra. I've always found SQL to be extremely verbose (the syntax reminds me of idk COBOL or smth) but there's another query language called Datalog, that's for our purposes similar to SQL but the syntax is much more legible.
E.g. check out these snippets from these slides (page 29) (and check out the whole class too)
s(X) <- p(X,Y).
s(X) <- r(Y,X).
t(X,Y,Z) <- p(X,Y), r(Y,Z).
w(X) <- s(X), not q(X).
CREATE VIEW s AS (SELECT a FROM p)
(SELECT b FROM r);
CREATE VIEW t AS
SELECT a, b, c
FROM p, r
WHERE p.b = r.a,
CREATE VIEW w AS
If you want working with SQL to become extremely easy to you, you will need to start creating websites, bring traffic to those sites and implement the scaling procedures you have learned from the books below.
As if you go to a job interview and want to blow it out of the water it is to your best advantage to know for example MySQL in and out so well you make the interviewer smile inside and go yes we have found the one. You will probably see a little smirk as the interviewer is trying to hold in their smile when this happens but you will know when this occurs very easily. Upside to this is you will be very comfortable getting started on day one and the only training you will need is the current architecture and backup or non existent backup plans in place so you can get to work.
I recommend the following hardcopy books in order:
If your wanting to do Microsoft SQL Server, I recommend the following:
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