What is the difference between UNION and UNION ALL?
In SQL, both `UNION` and `UNION ALL` operators are used to combine the result sets of two or more `SELECT` statements into a single result set. While their primary goal is similar, they differ significantly in how they handle duplicate rows and, consequently, their performance characteristics. Understanding these distinctions is crucial for efficient query writing and data manipulation.
Understanding UNION
The UNION operator combines the result sets of two or more SELECT statements and eliminates duplicate rows from the final result. For UNION to work, each SELECT statement must have the same number of columns, and the corresponding columns must have compatible data types. The column names in the final result set are usually taken from the first SELECT statement.
SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;Because UNION performs a distinct operation to remove duplicates, it often involves an implicit sorting or hashing process, which can make it slower and more resource-intensive, especially when dealing with large datasets. It guarantees a unique set of rows in its output.
Understanding UNION ALL
The UNION ALL operator combines the result sets of two or more SELECT statements, but unlike UNION, it retains all duplicate rows. This means if a row exists in both result sets, or multiple times within a single result set, it will appear as many times in the final output. Like UNION, the SELECT statements must have the same number of columns with compatible data types.
SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;Since UNION ALL does not perform any distinct operation or duplicate checking, it is generally much faster and less resource-intensive than UNION. It simply appends the results of subsequent SELECT statements to the first one, making it the preferred choice when you know there are no duplicates or when preserving duplicates is desired.
Key Differences Summarized
| Feature | UNION | UNION ALL |
|---|---|---|
| Duplicate Rows | Removes duplicates | Includes duplicates |
| Performance | Slower (due to duplicate removal) | Faster (no duplicate removal) |
| Sorting | Often involves implicit sorting/hashing | Does not involve implicit sorting/hashing |
| Resource Usage | Higher (more processing) | Lower (less processing) |
When to Use Which?
Use UNION when:
- You need to combine results from multiple queries and want only unique rows in the final output.
- You are intentionally filtering out duplicate data across your combined sets.
- The overhead of duplicate removal is acceptable for data integrity.
Use UNION ALL when:
- You need to combine results from multiple queries and want to retain all rows, including duplicates.
- Performance is a critical concern, and you are certain there are no duplicates, or duplicates are desired.
- You are simply aggregating all data from different sources without needing to de-duplicate.