SQL UNION ALL Operator - Complete Beginner Tutorial

What is UNION ALL?

UNION ALL is used to combine the results of two or more SELECT statements into a single result set.

It includes all rows from each query, even if some rows appear multiple times.

• Includes duplicates in the final result.
• Does not sort or remove duplicate rows.
• Requires each SELECT to have the same number of columns.
• Columns must have compatible data types.

Syntax of UNION ALL

The basic syntax of UNION ALL combines multiple SELECT statements separated by the UNION ALL keyword.

Example Usage of UNION ALL

Here is a practical example demonstrating how UNION ALL works.

Differences Between UNION and UNION ALL

Understanding the difference helps you choose the right operator for your needs.

• UNION removes duplicate rows, UNION ALL includes duplicates.
• UNION performs sorting and duplicate elimination, which can impact performance.
• UNION ALL is faster because it simply appends results.
• Use UNION when you need distinct results; use UNION ALL when duplicates are acceptable or performance is critical.

Best Practices and Performance Considerations

Using UNION ALL effectively can improve query performance and clarity.

• Use UNION ALL when you do not need to remove duplicates to improve speed.
• Ensure all SELECT statements have matching columns and compatible data types.
• Avoid unnecessary UNION ALL operations on large datasets without indexing.
• Consider indexing columns used in combined queries for better performance.