Learning SQL resources and PostGIS

http://developers.cartodb.com/tutorials/joining_data.html

A blog post geared towards a pay-for service called CartoDB, but explains roughly joining/relating geometry in Databases using SQL and the ST_Intersects function.

The Boundless Geo Tutorial Module Index

http://workshops.boundlessgeo.com/postgis-intro/index.html
An introduction to PostGIS, using the OpenGeo suite, which isn’t important, but the text might still be useful and it has a ton of recipes that could be adapted.

http://tech.pro/tutorial/1555/10-easy-steps-to-a-complete-understanding-of-sql

A rough intro to the way SQL “thinks” compared to other programming languages-The complete understanding claim is a bit of an exaggeration.

http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/

A blog post about SQL joins and getting Right, Left, Inner, and Outer joins straight.