What error occurs if key fields of a union in the Core Data Service View do not match upon activation?

When creating a Core Data Service (CDS) View that involves a union of multiple data sources, it is essential that the key fields match across all the data sources that are being combined. If there is a discrepancy in the key fields—meaning they do not have the same names, types, or structures—an activation error will occur.

In this scenario, the matched key fields are critical because they determine how records are uniquely identified and combined. If the key fields do not match upon activation, the system cannot reliably execute the union operation, leading to an error. This error ensures data integrity and consistency within the CDS view, preventing potential issues that could arise from misaligned identifiers.

This reinforces the importance of maintaining consistent key fields across all components involved in a union within CDS Views. Therefore, the answer indicating that an activation error occurs because the key fields of the union do not match is accurate and reflects a fundamental principle of working with CDS Views in SAP.