Understanding MySQL UNION vs UNION ALL: Key Differences and Performance

This article explains the differences between UNION and UNION ALL in MySQL, two commands used to combine the results of two or more SELECT statements.

Key Concepts

• UNION: Combines the result sets of two or more SELECT queries and removes duplicate rows.
• UNION ALL: Combines the result sets of two or more SELECT queries but retains all duplicate rows.

When to Use UNION

• Use UNION when you want to merge results from multiple queries and ensure that the final output contains only unique records.
• Example:

SELECT column1 FROM table1
UNION
SELECT column1 FROM table2;

When to Use UNION ALL

• Use UNION ALL if you want to include all results from the queries, including duplicates. This is often faster than UNION because it does not perform the duplicate elimination process.
• Example:

SELECT column1 FROM table1
UNION ALL
SELECT column1 FROM table2;

Performance Considerations

• Speed: UNION ALL is generally faster than UNION because it does not check for duplicates.
• Memory Usage: UNION may use more memory to store the unique results, while UNION ALL uses less since it does not filter duplicates.

Conclusion

Choose UNION for unique results and UNION ALL for complete results including duplicates. Understanding the differences helps in writing efficient SQL queries based on your data requirements.