UNION vs. UNION ALL: When Should You Use Each?

UNION and UNION ALL: A Detailed Comparison

In SQL database management, UNION and UNION ALL are powerful tools for merging result sets from multiple queries into a single table. While functionally similar, a key difference lies in their treatment of duplicate rows.

Understanding UNION ALL

UNION ALL directly concatenates rows from the source queries without any deduplication. This means duplicate rows are preserved in the final output. This is advantageous when complete data preservation is vital, such as during data extraction or the generation of comprehensive reports.

Understanding UNION

Unlike UNION ALL, UNION removes duplicate rows before combining the result sets. It performs a column-by-column comparison to identify and eliminate identical rows, resulting in a consolidated table containing only unique entries. This is ideal for scenarios requiring a deduplicated output, common in filtered reports or data analysis.

Performance Implications

While UNION offers the advantage of deduplication, it incurs a performance overhead compared to UNION ALL. The extra processing required for duplicate removal can significantly impact query execution time. Therefore, UNION ALL is generally preferred when duplicate rows are acceptable or when performance is critical.

Illustrative Examples

The following examples demonstrate the difference:

-- UNION ALL preserves duplicates
SELECT 'foo' AS bar
UNION ALL
SELECT 'foo' AS bar;

-- UNION removes duplicates
SELECT 'foo' AS bar
UNION
SELECT 'foo' AS bar;

The UNION ALL example yields two identical rows, whereas UNION produces a single row, effectively eliminating the duplicate.

By understanding the nuances of UNION and UNION ALL, database administrators can choose the optimal operator to achieve the desired outcome, whether it's preserving all data or ensuring a unique result set.

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