UNION ALL Operator

Overview

The UNION ALL operator combines the result sets of two or more SELECT queries without removing duplicate rows.

Learning Objectives

• Combine query outputs using UNION ALL.
• Understand why UNION ALL is faster than UNION.
• Preserve duplicate observations in stacked logs.

Detailed Concept Explanation

UNION ALL appends rows from multiple queries directly into a single result set without checking for duplicates or performing a sort. This makes UNION ALL significantly faster and more resource-efficient than UNION.

Code Examples

SQL

-- Combine historical and archive logs preserving every record
SELECT log_id, log_time, message FROM current_logs
UNION ALL
SELECT log_id, log_time, message FROM archive_logs;

Best Practices

• Default to UNION ALL: Make UNION ALL your default choice when stacking datasets unless duplicate suppression is strictly required.

Summary

UNION ALL stacks query result sets fast without performing duplicate suppression.