UNION vs. UNION ALL: What's the Difference?
Edited by Aimie Carlson || By Harlon Moss || Published on September 30, 2024
UNION combines results from two queries and removes duplicates, while UNION ALL combines results including all duplicates.
Key Differences
In SQL, UNION is used to combine the results of two or more SELECT statements, but it eliminates duplicate rows in the result set. On the other hand, UNION ALL also combines results from multiple SELECT statements, but it includes all duplicate rows.
UNION performs an additional step to remove duplicates, which can impact query performance, especially on large datasets. UNION ALL is faster since it does not check for duplicates and returns all rows from the combined queries.
The result set of UNION will always have unique rows, which can be important for data accuracy in certain queries. In contrast, UNION ALL may return a larger result set with duplicate rows, useful when duplicates are necessary or irrelevant.
UNION is ideal when needing a distinct result set, like in reports or data summaries where duplicates would distort results. UNION ALL is better suited for scenarios where all data, including repetitions, is needed, such as consolidating logs or records.
Both UNION and UNION ALL require that the SELECT statements involved have the same number of columns and compatible data types. The difference lies in their approach to handling duplicates.
ADVERTISEMENT
Comparison Chart
Duplicates Handling
Eliminates duplicates
Includes duplicates
Performance
Slower due to duplicate removal
Faster as no duplicate check is performed
Result Set
Only unique rows
All rows, including duplicates
Ideal Use Cases
When distinct results are required
When all data, including duplicates, is needed
Query Requirements
Same number and type of columns in SELECT
Same as UNION, with duplicate inclusion
ADVERTISEMENT
UNION and UNION ALL Definitions
UNION
Represents the set of all elements in either of two sets.
The union of sets A and B includes every element from both.
UNION ALL
In data processing, refers to the merging of datasets with all entries.
Applying UNION ALL ensured no data point was missed in the analysis.
UNION
A state of being united or joined.
The union of the two groups led to a stronger community.
UNION ALL
Emphasizes inclusion of every element, even if repeated, in a combined dataset.
UNION ALL was used to ensure every survey response was counted.
UNION
Combines results from multiple SELECT statements, removing duplicates.
Use UNION to merge two different customer lists into one without repeat entries.
UNION ALL
In data analysis, used for comprehensive data combination, including duplicates.
The report used UNION ALL to present a full picture of the responses.
UNION
An organized association of workers for collective bargaining.
The workers formed a union to negotiate better wages.
UNION ALL
Represents the total collection of elements from multiple sets, including repeats.
UNION ALL of these datasets includes every instance, repeated or not.
UNION
The act of marrying or joining in a close relationship.
Their union was celebrated with a grand wedding.
UNION ALL
Combines results from multiple SELECT statements, including duplicates.
Use UNION ALL to combine sales data from different regions, keeping all records.
UNION
The act of uniting or the state of being united.
FAQs
What does UNION do in SQL?
Combines results from multiple SELECT queries, removing duplicates.
Is UNION ALL faster than UNION?
Yes, because it doesn't perform duplicate removal.
When should I use UNION?
Use it when you need a distinct result set without duplicates.
Can UNION and UNION ALL be used interchangeably?
No, they serve different purposes based on whether duplicates are needed.
Are there any limitations to using UNION?
It can be slower on large datasets due to duplicate removal.
Can UNION ALL result in a larger dataset than UNION?
Yes, because it includes all duplicates.
How does UNION ALL differ from UNION?
UNION ALL combines results but includes all duplicates.
Does UNION ALL preserve the order of rows?
No, like UNION, it doesn't guarantee row order.
Are there performance considerations with UNION in large databases?
Yes, duplicate removal in UNION can be resource-intensive on large datasets.
Can UNION combine different data types in columns?
No, the data types need to be compatible in the columns being combined.
Do UNION and UNION ALL require the same columns in SELECT statements?
Yes, they both require the same number and type of columns.
How does UNION handle NULL values?
NULL values are treated as equivalent for duplicate removal.
Is UNION ALL suitable for data analysis?
Yes, especially when all data points, including duplicates, are important.
How do UNION and UNION ALL handle column names?
The column names in the result set are usually taken from the first SELECT statement.
Can UNION ALL be used for concatenating strings in SQL?
Not directly; it's for combining result sets, not individual string values.
Can UNION and UNION ALL be used in subqueries?
Yes, they can be used in subqueries.
What happens if the SELECT statements in UNION have different numbers of columns?
The query will return an error; the number of columns must match.
Are UNION and UNION ALL SQL standard operations?
Yes, they are part of the SQL standard.
Can UNION be used with more than two SELECT statements?
Yes, you can combine multiple SELECT statements with UNION.
Do UNION and UNION ALL have an impact on indexing?
They don't directly affect indexes, but indexes can affect their performance.
About Author
Written by
Harlon MossHarlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Aimie CarlsonAimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
