When using UNION to combine two SELECT statements, which condition must hold?

• A. Both queries must return the same number of columns in the same order with compatible data types.
• B. Both queries must select from the same table.
• C. The second query must not use GROUP BY.
• D. The results are always ordered by the first column.

Answer: (A)

When you combine two result sets with UNION, you’re stacking rows from both queries into one set. To do that cleanly, each SELECT must produce the same number of columns, in the same order, and the data types of corresponding columns must be compatible. This alignment lets the database merge the two results into a single, well-structured table where each column matches across both queries. You can alias columns to control the final names, and remember that duplicates are removed by default (use UNION ALL if you want to keep duplicates). The two queries don’t have to come from the same table, and there’s no requirement about GROUP BY, and the final ordering is determined by an ORDER BY at the end, not automatically by the first column.