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

UNION and UNION ALL: A Comparative Analysis of SQL Operators

Database systems utilize UNION and UNION ALL to merge result sets from multiple queries or tables. While functionally similar, a crucial difference dictates their appropriate use.

Key Distinguishing Feature:

The core difference lies in duplicate row handling. UNION eliminates duplicate rows where all column values match, ensuring a unique result set. UNION ALL, conversely, includes all rows, preserving duplicates.

Practical Implications:

Choosing between UNION and UNION ALL depends on your data processing needs. UNION is ideal when uniqueness is paramount, preventing redundant data in reports or analyses. UNION ALL, however, is better suited for scenarios requiring a complete, unfiltered view of the combined data, such as data aggregation from disparate sources.

Performance Impact:

UNION's duplicate removal incurs a performance overhead compared to UNION ALL. The database must process extra steps to identify and remove duplicates. This makes UNION less efficient for large datasets where performance is a primary concern.

Illustrative Example:

Consider these SQL queries:

<code class="language-sql">SELECT 'foo' AS bar
UNION
SELECT 'foo' AS bar</code>

UNION yields:

<code>+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)</code>

The duplicate is removed.

Now, using UNION ALL:

<code class="language-sql">SELECT 'foo' AS bar
UNION ALL
SELECT 'foo' AS bar</code>

The output is:

<code>+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)</code>

Duplicates are retained. This example clearly demonstrates the fundamental difference in their behavior.

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