SQL Union All

How do you combine the results of two or more SELECT statements?

UNION ALL combines the result sets of multiple SELECT statements into a single result set. It's crucial for aggregating data from different sources or tables. Crucially, it preserves all rows from the combined queries, including duplicates.

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Description

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Example H3

The UNION ALL operator in SQL is a powerful tool for combining the output of multiple SELECT statements. Imagine you have two tables: one containing customer orders from the East region and another for the West region. Using UNION ALL, you can seamlessly merge the data from both tables into a single result set, allowing you to analyze all orders regardless of their origin. This is particularly useful when you need to aggregate data from different sources or tables that share a similar structure. For instance, you might use it to combine data from different databases or data warehouses. The key difference between UNION and UNION ALL is that UNION automatically removes duplicate rows, while UNION ALL preserves them. This distinction is important because sometimes you need to see all the data, even if some rows are repeated. Understanding when to use each is crucial for effective data analysis.

Why SQL Union All is important

UNION ALL is essential for data aggregation and analysis. It allows you to combine data from various sources into a single dataset, enabling comprehensive insights. This is a fundamental operation for data warehousing and reporting.

SQL Union All Example Usage


-- Example using a hypothetical syntax checker (replace with your actual tool)

-- Valid SQL query
SELECT * FROM employees WHERE department = 'Sales';

-- Incorrect SQL query (missing semicolon)
SELECT * FROM customers
WHERE city = 'New York'

-- Incorrect SQL query (invalid column name)
SELECT * FROM employees WHERE department = 'Saless';

-- Incorrect SQL query (missing closing parenthesis)
SELECT * FROM orders WHERE order_date > '2023-01-01' AND order_total >

SQL Union All Syntax

Common Mistakes

Forgetting the `ALL` keyword when you need to preserve duplicates. Using `UNION` instead of `UNION ALL` when duplicates are desired.
Assuming the columns in the SELECT statements are identical. UNION ALL requires the columns to have compatible data types and order.

Frequently Asked Questions (FAQs)

When should I choose UNION ALL instead of UNION?

Use UNION ALL when you need a complete, unfiltered view of your data and want to preserve duplicate rows. For example, combining order tables from the East and West regions with UNION ALL keeps every record intact, enabling accurate counts and raw analysis. In contrast, UNION removes duplicates, which can hide important details if the same order appears in both tables.

Does UNION ALL run faster than UNION?

Yes. Because UNION ALL simply appends result sets without running the costly DISTINCT step required by UNION, it generally executes faster—especially on large datasets. Skipping duplicate elimination means the database engine avoids extra sorting or hashing operations, resulting in shorter run times and lower resource consumption.

How can Galaxy help me write and manage UNION ALL queries across multiple data sources?

Galaxy’s modern SQL editor streamlines writing UNION ALL statements with intelligent autocomplete, parameterization, and AI-powered query suggestions. If your tables live in different databases, you can connect each source, draft your UNION ALL query in a single tab, and let Galaxy’s context-aware copilot validate column alignment. Built-in collaboration and version history also make it easy to share and endorse the final query with your team—no more pasting SQL into Slack or Notion.