This repo houses a Merge / Join guide I wrote for the Center for Research Methods and Data Analysis (CRDMA) at the University of Kansas back when I was working there during my Ph.D. studies. This guide is way too xomprehensive for the topic, but it is useful for someone new to the topic.
Just download the PDF or HTML if you want to check out the guide, or clone the repo and Sweave the .Rnw file. I made this back when I was toying with the Sweave format and have since migrated to .Rmd for this kind of work.