A non-technical friend recently asked me for help with a merge problem. They had two separate data pulls of electronic medical records based on specific study parameters. The set of people in the database who fit the study parameters changed in between the data pulls, for example by having people age into our out of a study, or by having new diagnoses added to their records that cause them to either be newly included or excluded.
I frequently find myself needing to concatenate data sets but also wanting to be able to distinguish which row came from which data set originally. Introductory SAS courses tend to teach the in keyword, for a workflow similar to this:
A popular beginners machine learning problem is the prediction of housing prices. A frequently used data set for this purpose uses housing prices in California along some additional gathered through the 1990 Census.