Joining DataFrames

To demonstrate joining, we will use two CSV files: dest.csv and tips.csv. The use case behind it is that we are running a taxi company. Every time a passenger is dropped off at his or her destination, we add a row to the dest.csv file with the employee number of the driver and the destination:

EmpNr,Dest
5,The Hague
3,Amsterdam
9,Rotterdam

Sometimes drivers get a tip, so we want that registered in the tips.csv file (if this doesn't seem realistic, please feel free to come up with your own story):

EmpNr,Amount
5,10
9,5
7,2.5

Database-like joins in pandas can be done with either the merge() function or the join() DataFrame method. The join() method joins on indices by default, which might not be what you want. In SQL—a relational database query language—we have the inner join, left outer join, right outer join, and full outer join.

Note

An inner join selects rows from two tables, if and only if values match, for columns specified in the join condition. Outer joins do not require...