The OpenGeo (now called Boundless) PostGIS tutorial has long been my recommendation for people just getting started with PostGIS. It provides a broad intro to PostGIS, great for users new to SQL.
Unfortunately they recently removed it from their website. The guide was a fairly old now, so I don’t blame them. But it was a great free resource.
I have created a version of the data used, which can be accesses at (choose the green Clone or download button and Download ZIP):
The data is also not 100% the same as the original. So some of the queries will have to be modified. Particularly the neighborhoods are different. So they may have slightly different names/boundaries/capitalization. So keep that in mind when creating your queries, and interpreting the results. But overall the data is pretty similar
The course has been compiled and can be found at: https://gisforthought.com/boundless-postgis-tutorial/