Found in 2 comments on Hacker News
jklowden · 2015-02-09 · Original thread
I like Date's SQL and Relational Theory ( It won't tell you about the latest changes to the standard, but it will help you get a handle on the relational model, which is the key to using SQL intelligently.
AlisdairO · 2012-05-04 · Original thread
Depends on what you're into, but from the database angle:

Learning SQL is a really excellent intro, and probably covers the majority of what your average developer is going to ever need to know about SQL ( )

SQL and Relational Theory gives you more background on relational theory - basically gives a more academic background for existing practitioners of SQL. Date is pretty dry author with a few hangups, but it's largely good solid stuff ( )

Oracle Essentials is a good solid introduction to Oracle, and gives a useful bit of background on how database management systems work ( )

For other subjects:

Code Complete needs little introduction ( )

I've only just started reading Version Control with Git, but it seems pretty good so far, and is well-reviewed ( )

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