Arunav Goswami
Data Science Consultant at almaBetter
Explore the key differences between UNION and UNION ALL. Understand their use cases, performance impacts, error handling, and compatibility across SQL databases
When working with SQL databases, it's common to retrieve data from multiple tables using various techniques. One such technique involves combining the results of two or more queries. In SQL, there are two primary operators for this purpose: UNION and UNION ALL. Understanding what is the difference between UNION and UNION ALL is crucial for writing efficient and accurate SQL queries. This article delves into the distinctions, use cases, and performance considerations for each operator.
In SQL, the UNION operator is used to combine the results of two or more SELECT statements. It removes duplicate rows between the various SELECT statements, ensuring that the final result set contains only unique rows. Each SELECT statement within the UNION must have the same number of columns in the result sets with similar data types in the corresponding columns.
Syntax:
SELECT column1, column2, ... FROM table1 UNION SELECT column1, column2, ... FROM table2; |
The UNION ALL operator, like UNION, is used to combine the result sets of two or more SELECT queries into a single result set. However, unlike UNION, UNION ALL does not remove duplicates. It includes all rows from the queries, including duplicates.
Syntax:
SELECT column1, column2, ... FROM table1 UNION ALL SELECT column1, column2, ... FROM table2; |
Let's say we have two tables, Employees and Managers, and we want to create a list of all distinct names from both tables.
Employees Table:
EmployeeID | Name |
---|---|
1 | Alice |
2 | Bob |
3 | Charlie |
Managers Table:
ManagerID | Name |
---|---|
1 | Bob |
2 | Dave |
3 | Eve |
SELECT Name FROM Employees UNION SELECT Name FROM Managers; |
Result:
Name |
---|
Alice |
Bob |
Charlie |
Dave |
Eve |
This query will return a combined list of names from both tables without duplicates.
SELECT Name FROM Employees UNION ALL SELECT Name FROM Managers; |
Result:
Name |
---|
Alice |
Bob |
Charlie |
Bob |
Dave |
Eve |
Explanation:
Here's a tabular comparison of UNION and UNION ALL in SQL:
Feature | UNION | UNION ALL |
---|---|---|
Definition | Combines the results of two or more SELECT queries and removes duplicates. | Combines the results of two or more SELECT queries without removing duplicates. |
Duplicates Handling | Removes duplicate rows from the result set. | Keeps all duplicate rows in the result set. |
Performance | Slower due to the need to eliminate duplicates. | Faster because it does not need to check for duplicates. |
Use Case | Use when you need a distinct list of records. | Use when you need all records, including duplicates. |
Order of Results | Not guaranteed, as duplicates are removed; sorting may be needed. | Not guaranteed, but typically follows the order of the combined results. |
Result Set Size | Potentially smaller due to duplicate elimination. | Can be larger because it includes all duplicates. |
Memory Usage | Higher, as it needs to store and compare results to remove duplicates. | Lower, as it does not need additional memory for comparison. |
Use with Aggregates | Often used when combining results of aggregate functions where distinct results are required. | Suitable for aggregations where all records, including duplicates, are needed. |
Index Utilization | Can potentially use indexes less effectively due to the deduplication process. | Can leverage indexes more effectively as it simply appends results. |
Execution Plan Complexity | More complex execution plan due to duplicate removal process. | Simpler execution plan as it just combines the results. |
Data Integrity | Ensures data integrity by eliminating duplicates, which might be necessary for certain analytical tasks. | Maintains raw data integrity without alteration, suitable for raw data analysis. |
Potential Use Cases | Merging lists of unique values (e.g., combining customer IDs) | Combining logs or records where all entries, including repeats, are needed |
Check out our free resources to enhance your knowledge in SQL: SQL Cheat Sheet, online SQL compiler, and SQL tutorial.
When deciding between UNION and UNION ALL, performance is a significant factor. UNION’s additional step of removing duplicates can lead to higher CPU and memory usage, particularly with large datasets. Conversely, UNION ALL's lack of this step means it can handle larger volumes of data more efficiently.
To illustrate the performance difference, consider a database with millions of rows. Running a UNION query on such a dataset will take considerably longer than a UNION ALL query due to the overhead of deduplication. For high-performance applications, especially where duplicate rows are acceptable, UNION ALL is typically the preferred choice.
Both UNION and UNION ALL are powerful tools for combining data from multiple queries. The choice between them depends on your specific needs—whether you prioritize data uniqueness or performance. By understanding their differences and use cases, you can write more efficient and effective SQL queries. Always consider the nature of your data and the requirements of your application when deciding which operator to use.
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