When merging results of queries using the UNION keyword in a CDS view, what must be true about the column lists?

When using the UNION keyword in a CDS view to merge the results of multiple queries, it is essential that the involved query outputs have the same number of columns. This requirement ensures that the structure of the final output is consistent, allowing the database engine to align each column from the different queries in a single result set.

Having an equal number of columns is foundational for combining datasets with the UNION operator, as each result set is aligned by their respective column positions. If the queries do not have the same number of columns, the operation would not be valid and would lead to a runtime error. Thus, it's critical to ensure a matching column count for a successful merge of query results.

While compatibility of types and identical types among the columns are important considerations when working with UNION, the primary requirement is the same number of columns, which directly addresses the ability to execute the UNION operation itself.