Which single Google Cloud database should an App Engine application use when it must support transactional shopping and allow BI analysis across datasets in one database?

Cloud SQL is the correct choice because it's a fully managed relational database service that supports transactional workloads, which are essential for shopping applications (e.g., ensuring atomicity of orders). Its relational nature makes it suitable for complex queries and joins, facilitating Business Intelligence (BI) analysis directly on the operational data. BigQuery is an analytical data warehouse, not designed for transactional OLTP workloads. Cloud Bigtable is a NoSQL wide-column store, optimized for high-throughput, low-latency access to large datasets, but less ideal for complex transactional queries or BI analysis across diverse datasets within a single database. Cloud Datastore (now Firestore in Datastore mode) is a NoSQL document database, good for semi-structured data and flexible schemas, but not typically the primary choice for complex transactional systems requiring strong relational integrity and BI analysis in one database.