Set operations: does the UNION clause combine the outputs of multiple queries into a single result set?

Introduction / Context: UNION is one of SQL’s set operations (UNION, UNION ALL, INTERSECT, EXCEPT/MINUS). It vertically concatenates the results of multiple SELECT statements into a single result set. This question asks if that description is accurate.

Given Data / Assumptions:

• Each SELECT in a UNION must have the same number of columns and union-compatible types.
• UNION removes duplicates; UNION ALL keeps all rows.
• Column names are taken from the first SELECT unless aliased.

Concept / Approach: Conceptually, UNION stacks result sets. After stacking, UNION applies a duplicate elimination step, while UNION ALL does not. Ordering is optional and, when used, applies to the final combined set via a trailing ORDER BY clause.

Step-by-Step Solution:

Verification / Alternative check: Compare UNION vs UNION ALL on datasets with duplicates to see the effect on row counts.

Why Other Options Are Wrong:

• Incorrect: Misrepresents UNION’s role.
• Requiring identical column names / ORDER BY / UNION ALL is unnecessary; structural compatibility is what matters.

Common Pitfalls: Mismatched column counts; forgetting that ORDER BY must reference the combined output; assuming UNION preserves internal ordering of each branch.

Final Answer: Correct