What is the cardinality for connecting a customer data extension to a purchases data extension, where each customer can have multiple purchases?

The correct answer is one-to-many. In a scenario where each customer can have multiple purchases, the relationship between the customer data extension and the purchases data extension is characterized by the fact that a single customer record can be linked to many purchase records.

This means that for any individual customer, you can have multiple entries in the purchases data extension, each corresponding to a different purchase made by that customer. This model is essential for accurately representing and analyzing customer behavior, allowing marketers to track the purchasing history of each customer without duplicating customer information in the purchases data extension.

In a one-to-one relationship, each record in the first data extension would directly correspond to only one record in the second, which isn’t applicable here since one customer can make many purchases. Meanwhile, a many-to-many relationship would imply that multiple customers can have multiple purchases, complicating the direct link and typically requiring a junction table to manage the associations. The many-to-one relationship would also indicate that multiple purchases link back to a single customer in each case, but it fails to recognize the multiple purchase records for each customer in the context described.