What is required when defining a CORE data services (CDS) view using the UNION query?

When defining a CORE Data Services (CDS) view using a UNION query, it is essential that columns have compatible types. This means that even if the columns across the different SELECT statements in the UNION are not of the exact same data type, they should be able to convert from one type to another without data loss or error. For example, a string type can often be unioned with a character type, as they are compatible, while a numeric type might be paired with an integer type.

This compatibility ensures that the database can effectively combine the results from both queries into a single dataset that maintains consistency. It allows for flexibility in terms of how the data is structured, aiding developers in constructing effective data models that pull relevant information from various sources without extensive type conversions.

The requirement for all key fields to be included is not necessarily true for unions; while including them can be important for data integrity, it isn't a strict requirement for the success of a UNION operation. Similarly, the stipulations about having identical types or the same number of columns, while they might apply in some contexts, are not as flexible when it comes to unions that focus on compatible types for the purpose of merging datasets.