SQL Union Vs Union All

What's the difference between UNION and UNION ALL in SQL?

UNION and UNION ALL combine the results of two or more SELECT statements. UNION removes duplicate rows, while UNION ALL returns all rows, including duplicates. Choosing the correct one depends on whether you need to eliminate redundancy.

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Description

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In SQL, the UNION and UNION ALL operators are used to combine the result sets of multiple SELECT statements into a single result set. They are crucial for tasks like merging data from different tables or views. The key difference lies in how they handle duplicate rows. UNION, by default, eliminates duplicate rows from the combined result set. This is useful when you want a unique list of values. Think of it as finding the unique values across multiple tables. UNION ALL, on the other hand, returns all rows from the combined result sets, including duplicates. This is more efficient if you need all the data, even if some values appear multiple times in the different SELECT statements. Imagine you need to aggregate data from multiple sources, and you want to keep all the records, even if some have identical values.Understanding the difference is critical for writing efficient and accurate queries. Using UNION when you don't need to eliminate duplicates can lead to unnecessary processing and potentially incorrect results. Conversely, using UNION ALL when you need unique results will result in redundant data, which can be inefficient.

Why SQL Union Vs Union All is important

Understanding UNION and UNION ALL is essential for data manipulation and analysis. It allows you to combine data from different sources while controlling for duplicates, leading to more accurate and efficient queries. This is a fundamental skill for any SQL developer.

SQL Union Vs Union All Example Usage


-- Declare a table variable
DECLARE @Customers TABLE (
    CustomerID INT,
    FirstName VARCHAR(50),
    City VARCHAR(50)
);

-- Insert data into the table variable
INSERT INTO @Customers (CustomerID, FirstName, City)
SELECT CustomerID, FirstName, City
FROM Customers
WHERE City = 'New York';

-- Select data from the table variable
SELECT * FROM @Customers;
-- Example of using it in a calculation
SELECT AVG(CustomerID) AS AverageCustomerID FROM @Customers;

SQL Union Vs Union All Syntax

Common Mistakes

Forgetting to use UNION ALL when duplicates are expected.
Using UNION when you need all rows, including duplicates.
Incorrectly assuming that UNION automatically sorts the results.

Frequently Asked Questions (FAQs)

When should I choose UNION instead of UNION ALL in my SQL queries?

Use UNION when you specifically need a deduplicated result set. Because UNION removes duplicate rows across all participating SELECT statements, it is ideal for generating a unique list of IDs, emails, or any field where each value must appear only once. If your downstream logic (for example, a DISTINCT count or a lookup table) relies on uniqueness, UNION is the safer option, even though it performs an additional duplicate-elimination step.

Does UNION ALL really outperform UNION, and why?

Yes. UNION ALL generally runs faster than UNION because the database engine can simply append the result sets together without performing the expensive sort/hash step required to weed out duplicates. If you know that your SELECT statements already return unique rows—or you are intentionally preserving duplicates for aggregation—UNION ALL avoids unnecessary processing and can lead to significant speed improvements, especially on large data sets.

How can Galaxy’s AI copilot help me decide between UNION and UNION ALL?

Galaxy’s context-aware AI copilot analyzes your query in real time and surfaces recommendations. If it detects a UNION that could safely be changed to UNION ALL, it will suggest the swap to improve performance. Conversely, if you use UNION ALL but reference the combined result in a SELECT DISTINCT or GROUP BY that implies you really needed unique rows, Galaxy highlights the potential data-quality issue. This guidance lets you write efficient, correct SQL without manual trial and error.