Sql Union Union All

Understanding the Key Differences Between UNION and UNION ALL in SQL

When working with SQL databases, you may come across the terms UNION and UNION ALL. These two operators are essential in the world of databases, allowing you to combine the results of multiple SELECT queries. However, it’s crucial to understand how they differ to utilize them effectively in your SQL scripts. In this blog post, we will delve into the differences between UNION and UNION ALL, explore their functionalities, and provide practical examples.

What is UNION?

The UNION operator is used to combine the results of two or more SELECT statements into a single result set. The key characteristic of UNION is that it eliminates duplicate rows from the results. When you use UNION, the database engine automatically checks for duplicate entries and returns only unique rows.

Key Features of UNION:

  • Removes Duplicate Rows: Ensures that the final result includes only unique entries.
  • Performance Impact: There can be a performance cost since the database has to process and eliminate duplicates.

Example of UNION:

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

Result:

+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)

What is UNION ALL?

On the other hand, UNION ALL also combines the results of two or more SELECT statements, but it includes all rows, regardless of duplicates. This means if multiple SELECT statements return the same row, UNION ALL will display them all in the final result.

Key Features of UNION ALL:

  • Includes Duplicates: There are no eliminations based on duplicate rows, leading to potentially larger result sets.
  • Better Performance: Generally faster than UNION because it does not need to conduct the additional task of removing duplicates.

Example of UNION ALL:

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

Result:

+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)

When to Use UNION vs. UNION ALL?

  • Use UNION when you want to ensure that your results are free from duplicates, which is particularly important in report generation where unique entries may be necessary.
  • Opt for UNION ALL when you want to maximize performance, and retrieving duplicate rows is acceptable or even desired in your query.

Performance Considerations

It’s important to consider performance when choosing between UNION and UNION ALL. Using UNION can incur a performance hit since the SQL server must perform additional work to identify and remove duplicates. In contrast, UNION ALL operates with less overhead since it simply combines results.

Conclusion

Understanding the difference between UNION and UNION ALL can significantly impact your SQL query performance and result accuracy. While both operators are useful for data manipulation, choosing the appropriate one based on your needs will enhance your database operation efficiency. Remember to assess your data and reporting requirements before deciding on which operator to use in your SQL queries.

By keeping these differences in mind, you can make more informed decisions and write better-performing SQL queries. Happy querying!