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

SQL UNION and UNION ALL: A Comparative Analysis

In SQL, UNION and UNION ALL are set operators used to combine result sets from multiple SELECT statements. The key difference lies in their handling of duplicate rows.

UNION Operator:

The UNION operator merges result sets, removing duplicate rows in the process. It compares all columns across the combined datasets and only includes unique row combinations in the final output. This deduplication step adds computational overhead.

UNION ALL Operator:

UNION ALL also combines result sets, but it retains all rows, including duplicates. This means it simply concatenates the results without any filtering. Consequently, UNION ALL generally executes faster than UNION because it avoids the duplicate elimination step.

Performance Implications:

While UNION offers the benefit of data deduplication, it impacts performance. The database system must perform a comparison and filtering operation to remove duplicates. If duplicate rows aren't a concern, or if performance is paramount, UNION ALL is the more efficient choice.

Illustrative Examples:

Let's examine the behavior of each operator with a simple example:

UNION Example:

SELECT 'foo' AS bar
UNION
SELECT 'foo' AS bar;

Output:

<code>+-----+
| bar |
+-----+
| foo |
+-----+
1 row</code>

UNION ALL Example:

SELECT 'foo' AS bar
UNION ALL
SELECT 'foo' AS bar;

Output:

<code>+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows</code>

Choosing the Right Operator:

The selection between UNION and UNION ALL depends on the specific needs of your query. If eliminating duplicates is essential for data accuracy, UNION is preferred. However, if performance is a priority and duplicate rows are acceptable, UNION ALL is the better option.

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