Union In SQL

How can I combine the results of two or more SELECT statements?

The UNION operator in SQL combines the result sets of two or more SELECT statements into a single result set. It's crucial for merging data from different sources or filtering results from multiple queries. Crucially, it removes duplicate rows.

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Description

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Example H3

The UNION operator is a powerful tool in SQL for combining the output of multiple SELECT statements. Imagine you have two tables: one containing customer orders and another containing customer returns. You might want to see all orders and returns in a single view. UNION allows you to do precisely that. It takes the result sets from each SELECT statement and stacks them vertically, creating a single, unified result. A key aspect of UNION is that it automatically eliminates duplicate rows, ensuring data integrity in the combined result. This is particularly useful when dealing with data from multiple sources that might have overlapping entries. For example, if both SELECT statements return a row with the same customer ID, UNION will only include that row once. This is different from the UNION ALL operator, which includes all rows, even duplicates. Understanding the difference between UNION and UNION ALL is essential for accurate data manipulation.

Why Union In SQL is important

UNION is essential for data analysts and developers who need to synthesize information from various sources. It simplifies complex queries and ensures data accuracy by eliminating redundant rows. This makes it a fundamental tool for creating comprehensive reports and analyses.

Union In SQL Example Usage


CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE
);

CREATE TABLE Returns (
    ReturnID INT PRIMARY KEY,
    CustomerID INT,
    ReturnDate DATE
);

INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES
(1, 101, '2023-10-26'),
(2, 102, '2023-10-27'),
(3, 101, '2023-10-28');

INSERT INTO Returns (ReturnID, CustomerID, ReturnDate) VALUES
(1, 101, '2023-10-27'),
(2, 103, '2023-10-29');

SELECT CustomerID, OrderDate FROM Orders
UNION
SELECT CustomerID, ReturnDate FROM Returns;

Union In SQL Syntax

Common Mistakes

Forgetting to use the same column data types in the SELECT statements. The columns in each SELECT statement must have compatible data types for UNION to work correctly.
Using UNION when UNION ALL is appropriate. If you need all rows, including duplicates, use UNION ALL instead of UNION.
Incorrectly ordering the columns in the SELECT statements. The columns in each SELECT statement must be in the same order for UNION to work correctly.

Frequently Asked Questions (FAQs)

Why use the SQL UNION operator instead of running separate SELECT statements?

UNION lets you merge the outputs of multiple SELECT queries into one unified result set, saving you from manually stitching data together in application code or spreadsheets. By vertically stacking the rows from each query, you can instantly analyze all related records—such as orders and returns—side by side, apply one ORDER BY or LIMIT clause to the whole set, and share a single, cleaner dataset with teammates.

What is the difference between UNION and UNION ALL when it comes to duplicate rows?

UNION automatically removes duplicate rows that appear in more than one SELECT statement, ensuring that each distinct record shows up only once in the final output. In contrast, UNION ALL keeps every row, even if it appears multiple times, which makes it faster but can lead to inflated counts or misleading analyses if duplicates exist. Choose UNION when data integrity and de-duplication are priorities; choose UNION ALL when you need raw, unfiltered results and maximum performance.

How can Galaxy help me write and manage UNION or UNION ALL queries?

Galaxy’s context-aware AI copilot autocompletes table names, suggests correct column alignments, and warns you if your SELECT statements have mismatched column counts—common pitfalls when building UNION queries. You can version, share, and “Endorse” finished UNION or UNION ALL statements in Galaxy Collections so teammates reuse the exact query instead of pasting snippets in Slack, ensuring consistent, duplicate-free reporting across your organization.