When a database administrator (DBA) uses the SQL command UNION to combine tables, what action does this represent?

• A. Append rows
• B. Append columns
• C. Merge databases
• D. Cross join tables

Answer: (B)

The use of the SQL command UNION effectively allows the combination of rows from two or more tables that have the same number of columns and compatible data types. It is important to understand that when UNION is employed, it aggregates data from these distinct tables into a single result set, thereby appending the rows of the second table to those of the first table.

The action represented by UNION is fundamentally about combining data across rows. If different tables contain similar datasets, UNION is particularly useful for unifying their results into one cohesive output. Additionally, UNION inherently removes any duplicate rows from the resulting set unless you use UNION ALL, which distinctly retains all records.

The other options do not reflect the functionality of UNION accurately. For instance, appending columns pertains to a different operation such as a JOIN, which merges vertical structures. Merging databases is a more comprehensive process that could involve physically combining two databases rather than merely combining rows for query results. A cross join, on the other hand, produces a Cartesian product between tables, creating combinations of rows rather than appending them.

Through understanding this, it’s clear that the correct action represented by the SQL UNION command is to append rows from multiple tables into a unified dataset.