UNION vs UNION ALL: Key Differences Explained

The fundamental difference between UNION and UNION ALL in SQL is that UNION removes duplicate records from the final result set, while UNION ALL retains all rows including duplicates. UNION performs an additional deduplication step after combining the data, making it slower but ensuring unique results, whereas UNION ALL simply concatenates the results without any duplicate removal, offering better performance.

Contents

• Basic Definitions and Syntax
• Key Differences Explained
• Performance Considerations
• When to Use Each Operator
• Practical Examples
• Common Use Cases

Basic Definitions and Syntax

Both UNION and UNION ALL are set operators in SQL that combine the results of two or more SELECT statements into a single result set. The basic syntax structure is identical:

SELECT column1, column2 FROM table1
UNION [ALL]
SELECT column1, column2 FROM table2;

Key Requirements:

• Both SELECT statements must have the same number of columns
• The corresponding columns in each SELECT statement must have compatible data types
• Column names can differ, but the data types must be compatible
• The ORDER BY clause can only be used once at the very end of the entire UNION/UNION ALL statement

Key Differences Explained

The primary distinction lies in how each operator handles duplicate records:

UNION Operator

• Removes duplicates: After combining the result sets, UNION performs a deduplication step
• Sorts results: Typically requires sorting operations to identify and eliminate duplicate rows
• Returns distinct records: Each row in the final result set is unique
• Slower performance: The additional deduplication process makes it slower than UNION ALL

UNION ALL Operator

• Retains duplicates: Combines all rows from both result sets without any filtering
• No sorting required: Simply concatenates the results without additional processing
• Returns all records: Includes every row from both SELECT statements
• Faster performance: Avoids the overhead of duplicate detection and removal

According to Atlassian’s SQL documentation, “UNION performs a deduplication step before returning the final results, UNION ALL retains all duplicates and returns the full, concatenated results.”

Performance Considerations

The performance difference between UNION and UNION ALL is significant, especially with large datasets:

Performance Characteristics

When UNION Might Be Faster

Interestingly, there are scenarios where UNION can outperform UNION ALL. As noted in Stack Overflow discussions, “I’m doing some performance tweaking right now and finding UNION almost twice as fast as UNION ALL even though the resulting query returns the exact same set of results.”

This typically occurs when:

• The database optimizer can use more efficient indexing strategies
• There are very few duplicates to remove
• The data is already sorted in a way that facilitates deduplication

As StrataScratch explains, “Because UNION takes the extra step of removing duplicate values, generally, it is considered slower than UNION ALL, but that’s not always the case.”

When to Use Each Operator

Use UNION When:

• You need distinct results without any duplicate records
• Business logic requires unique values in the final output
• Data integrity is more important than performance
• The datasets are relatively small (performance difference is negligible)

Use UNION ALL When:

• You need all rows including duplicates

• Performance is a priority and duplicates are acceptable

• You know there are no duplicates in the result sets

• Working with large datasets where performance matters significantly

• DataCamp’s tutorial states: “If you need a distinct result set without duplicates, use UNION. If you want to include all rows, including duplicates, and prioritize performance, use UNION ALL.”

Practical Examples

Example 1: Basic Usage

Tables:

Table_A:
ID  Name
1   Alice
2   Bob
3   Charlie

Table_B:
ID  Name
2   Bob
4   David
5   Eve

UNION Query:

SELECT ID, Name FROM Table_A
UNION
SELECT ID, Name FROM Table_B;

Result:

ID  Name
1   Alice
2   Bob
3   Charlie
4   David
5   Eve

UNION ALL Query:

SELECT ID, Name FROM Table_A
UNION ALL
SELECT ID, Name FROM Table_B;

Result:

ID  Name
1   Alice
2   Bob
3   Charlie
2   Bob
4   David
5   Eve

Example 2: Performance Optimization

When working with large datasets, consider filtering data before the union operation:

-- Less efficient: filter after union
SELECT customer_id, order_date FROM orders
UNION
SELECT customer_id, order_date FROM returns
WHERE order_date > '2023-01-01';

-- More efficient: filter before union
SELECT customer_id, order_date FROM orders
WHERE order_date > '2023-01-01'
UNION ALL
SELECT customer_id, order_date FROM returns
WHERE order_date > '2023-01-01';

As SQLPad.io suggests: “To optimize such queries, consider filtering data as much as possible before the union operation. This reduces the workload during the duplicate removal process, striking a balance between data integrity and performance.”

Common Use Cases

UNION Applications:

• Creating comprehensive lists of unique customers across multiple systems
• Generating distinct product catalogs from different suppliers
• Building consolidated reports where duplicates would skew analytics
• Data warehousing scenarios requiring unique dimension records

UNION ALL Applications:

• Combining time-series data from multiple periods
• Creating comprehensive audit trails
• Merging transaction logs from different systems
• Building complete customer interaction histories
• Aggregating clickstream data for analytics

Conclusion

The choice between UNION and UNION ALL in SQL depends on your specific requirements for data uniqueness and performance. UNION ensures distinct results by removing duplicates but at the cost of additional processing time, while UNION ALL delivers all rows with optimal performance. When working with large datasets or when duplicates are acceptable, UNION ALL is typically the better choice. However, when data integrity requires unique values, the performance overhead of UNION is justified for ensuring accurate results.

Key Recommendations:

1. Use UNION when duplicate records would compromise your analysis
2. Choose UNION ALL for performance-critical operations with large datasets
3. Consider filtering data before union operations to improve performance
4. Test both approaches with your specific dataset to determine optimal performance
5. Be aware that database optimizers may sometimes produce unexpected performance results

Sources

1. UNION vs UNION ALL in SQL | Atlassian
2. Difference Between UNION and UNION ALL - GeeksforGeeks
3. SQL UNION vs UNION ALL - Syntax, Differences & Examples | DevArt
4. UNION vs UNION ALL in SQL | DataCamp
5. SQL UNION vs UNION ALL: Differences You Need to Know | StrataScratch
6. Union vs Union All - Usage and Performance In SQL Server | My Tec Bits
7. Union vs Union All in SQL: Key Differences Explained | SQLPad.io
8. Understanding the Difference Between UNION and UNION ALL in SQL | Medium