the data science process client item month client region john doe coca-cola january john doe ny jackie qi pepsi-cola january jackie qi nc client item month region john doe coca-cola january ny jackie qi pepsi-cola january nc figure joining two tables on the item and region keys are called primary keys one table may have buying behavior and the other table may have demographic information on person in figure both tables contain the client nameand this makes it easy to enrich the client expenditures with the region of the client people who are acquainted with excel will notice the similarity with using lookup function the number of resulting rows in the output table depends on the exact join type that you use we introduce the different types of joins later in the book appending tables appending or stacking tables is effectively adding observations from one table to another table figure shows an example of appending tables one table contains the observations from the month january and the second table contains observations from the month february the result of appending these tables is larger one with the observations from january as well as february the equivalent operation in set theory would be the unionand this is also the command in sqlthe common language of relational databases other set operators are also used in data sciencesuch as set difference and intersection client item month client item month john doe coca-cola january john doe zero-cola february jackie qi pepsi-cola january jackie qi maxi-cola february client item month john doe coca-cola january jackie qi pepsi-cola january john doe zero-cola february jackie qi maxi-cola february figure appending data from tables is common operation but requires an equal structure in the tables being appended