Combining datasets using a left or right join

In the Combining datasets using an inner join recipe, we combined data that was complementary and complete, orders, and associated customer information. However you may find a use case where complementary data is present for some records, and not for others. An example of this that we'll use in this recipe involves two data sources, one with sales data from a department store, and another data source with information from customers who checked out with a loyalty card. Of course, not all customers may have a loyalty card, and so we cannot expect to match every row in the data. This is where a left join comes into play.

In a left join, we pass through all the data from the first dataset, that is, the left data source, and only those records from the second, right data source that we were able to match. This means that any sales records that did not involve a loyalty card will still pass through, but the additional fields from the...