Pandas populate new dataframe column based on matching columns in another dataframe

Question

I have a df which contains my main data which has one million rows. My main data also has 30 columns. Now I want to add another column

Answer 1

While the other answers here give very good and elegant solutions to the asked question, I have found a resource that both answers this question in an extremely elegant fashion, as well as giving a beautifully clear and straightforward set of examples on how to accomplish join/ merge of dataframes, effectively teaching LEFT, RIGHT, INNER and OUTER joins.

Join And Merge Pandas Dataframe

I honestly feel any further seekers after this topic will want to also examine his examples...