"What books you bought on Amazon is stored in an Oracle database." This has not been true for some time. https://www.theregister.com/2019/10/16/amazon_ditches_oracle...
Amazon still runs on Oracle ERP. Even though they removed Oracle databases from AWS and other internal orgs, their own financials are still run in Oracle EBS. Thus the data for all the books bought by a person might still be in Oracle
Black Friday, and Oracle's inability to scale up to meet it without explosive licensing costs, is a major factor in Amazon's decision to remove Oracle from their sales systems.