In SQL, UNION and UNION ALL are used to combine the results of two or more SELECT queries into a single result set. While they serve a similar purpose, there are significant differences in their functionality, performance, and how they handle duplicate rows.
This article explores the key differences between UNION and UNION ALL, their syntax, and when to use each.
What is UNION?
UNION is used to combine the results of two or more SELECT statements while automatically removing duplicate rows from the output. The result set will contain only unique rows.
Syntax:
Key Features of UNION:
- Eliminates Duplicates: Ensures that no duplicate rows are present in the result set.
- Sorts Data: By default, the result is sorted in ascending order (based on the first column).
- Performance Impact: Since it removes duplicates, UNION requires additional computation, which can make it slower than UNION ALL, especially for large datasets.
What is UNION ALL?
UNION ALL combines the results of two or more SELECT statements without removing duplicate rows. The result set includes all rows from the input queries, including duplicates.
Syntax:
Key Features of UNION ALL:
- Retains Duplicates: Does not check for duplicate rows, making it faster than UNION.
- No Sorting: The result set is not sorted unless explicitly specified using an
ORDER BYclause. - Better Performance: Since it skips the duplicate-checking step, UNION ALL performs better, especially with large datasets.
Differences Between UNION and UNION ALL
| Aspect | UNION | UNION ALL |
|---|---|---|
| Duplicate Handling | Removes duplicates from the result set. | Includes all rows, even duplicates. |
| Performance | Slower due to duplicate elimination. | Faster as it doesn’t perform duplicate checks. |
| Sorting | Automatically sorts the result set. | Does not sort unless specified explicitly. |
| Use Case | When duplicate rows are not desired. | When duplicates are acceptable or required. |
| Output Size | Smaller output as duplicates are removed. | Larger output as all rows are included. |
When to Use UNION vs. UNION ALL?
Use UNION When:
- You want only unique rows in the result set.
- Duplicate data would cause confusion or inaccuracies in the results.
- The dataset is relatively small, so performance is not a major concern.
Use UNION ALL When:
- You need all rows, including duplicates, such as when analyzing raw data.
- Performance is critical, and the duplicate removal step would be too costly.
- Duplicates are meaningful and should not be discarded.
Example
UNION Example:
- This query combines names from both tables but removes any duplicate names.
UNION ALL Example:
- This query combines names from both tables and includes duplicate names if they exist.
While both UNION and UNION ALL are used to combine query results, they differ significantly in how they handle duplicates and their impact on performance. Choosing between the two depends on your specific use case. Use UNION if you need unique results, and opt for UNION ALL if duplicates are acceptable and performance is a priority. Understanding these differences will help you write efficient SQL queries tailored to your requirements.