Learning SQL cover
Learning SQL
by Alan Beaulieu
ISBN: 9780596007270
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AlisdairO · 2017-06-13 · Original thread
(self plug) for learning Postgres-flavoured SQL, take a look at https://pgexercises.com.

Books-wise, Learning SQL is a great start. http://shop.oreilly.com/product/9780596007270.do

The postgres docs themselves are also pretty fantastic.