UNION vs. UNION ALL — What's the Difference?
By Tayyaba Rehman — Published on January 28, 2024
UNION combines results from multiple SELECT statements, removing duplicates. UNION ALL also combines results but includes all duplicates, making it faster.
Difference Between UNION and UNION ALL
Table of Contents
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Key Differences
UNION and UNION ALL are SQL commands used to combine the results of two or more SELECT statements. The key difference lies in how they handle duplicates. UNION performs a distinct operation, which means it automatically removes duplicate rows from the results. UNION ALL, on the other hand, does not remove duplicates and includes all rows from the combined SELECT statements.
In terms of performance, UNION ALL is generally faster than UNION. This is because UNION requires additional processing to identify and remove duplicate rows, which can be time-consuming for large datasets. UNION ALL simply concatenates the results, skipping this deduplication step, thereby offering better performance, especially when working with large amounts of data.
The choice between UNION and UNION ALL depends on the specific requirements of the query. If the goal is to ensure no duplicate records in the result set, UNION is the appropriate choice. However, if the query's logic allows for duplicates, or if preserving the original dataset (including duplicates) is necessary, then UNION ALL is more suitable.
Both UNION and UNION ALL require that each SELECT statement within the query has the same number of columns in the result sets, and the corresponding columns must have compatible data types. However, because of the additional deduplication step, UNION may also require more memory and CPU resources compared to UNION ALL.
In summary, while both commands are used to combine results from different SELECT queries, UNION is used when distinct results are needed, and performance is not the primary concern. UNION ALL is preferable when performance is crucial, and duplicates in the result set are acceptable or desired.
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Comparison Chart
Duplicate Handling
Removes duplicates
Includes all duplicates
Performance
Slower, due to deduplication process
Faster, as no deduplication is performed
Use Case
When distinct results are required
When duplicates are acceptable or needed
Resource Usage
Requires more memory and CPU for deduplication
Less resource-intensive
Column Requirements
Same number and compatible types of columns required
Same number and compatible types of columns required
Compare with Definitions
UNION
Ensures distinct results from combined queries.
UNION is used to get a unique list of cities from two different tables.
UNION ALL
Suitable when duplicates are acceptable in results.
UNION ALL was used because duplicates in the report were necessary.
UNION
Useful for combining similar data from different sources.
UNION helped combine customer data from different regional databases.
UNION ALL
Less resource-intensive than UNION.
The query was more efficient with UNION ALL due to reduced processing.
UNION
Slower than UNION ALL due to deduplication.
The query took longer because UNION had to remove duplicate entries.
UNION ALL
Preferred for faster performance without deduplication.
UNION ALL is faster as it doesn't spend time removing duplicates.
UNION
Requires equal number of columns in SELECT statements.
UNION demands that both queries have the same structure for merging.
UNION ALL
Combines results from multiple SELECT statements, including duplicates.
SELECT column FROM table1 UNION ALL SELECT column FROM table2; keeps all duplicates.
UNION
Combines results from multiple SELECT statements, removing duplicates.
SELECT column FROM table1 UNION SELECT column FROM table2; combines results uniquely.
UNION ALL
Requires same number and type of columns in combined queries.
UNION ALL demands compatibility in column structure across all SELECT statements.
Common Curiosities
Is UNION ALL faster than UNION?
Yes, because it doesn't perform deduplication.
Do UNION and UNION ALL require the same number of columns?
Yes, the SELECT statements must have the same number and compatible types of columns.
What does UNION do in SQL?
It combines the results of two or more SELECT statements and removes duplicates.
When should you use UNION?
When you need to merge results and ensure no duplicates.
Can you use UNION ALL for distinct results?
No, it includes all duplicates; use UNION for distinct results.
How does UNION ALL differ from UNION?
UNION ALL also combines results but includes duplicates.
Is UNION suitable for large datasets?
It can be used, but it's slower due to deduplication, especially with large data.
Can UNION be used with different tables?
Yes, as long as the SELECT statements have compatible columns.
Does UNION ALL use more memory than UNION?
Generally, it uses less memory since it skips deduplication.
Can I combine more than two SELECT statements with UNION?
Yes, you can combine multiple SELECT statements using UNION.
How does UNION handle NULL values?
In UNION, NULL values are treated as duplicates of other NULLs.
Does UNION ALL keep the order of rows?
It concatenates rows but doesn’t guarantee order unless ORDER BY is used.
Can UNION be used for columns with different data types?
The data types must be compatible or convertible.
Are the results from UNION sorted?
UNION does not inherently sort results; an ORDER BY clause is needed for sorting.
Is UNION ALL a good choice for data analysis?
Yes, especially when analyzing complete data sets, including duplicates.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
