What is the difference between UNION and UNION ALL?

Learn what is the difference between union and union all? with practical examples, diagrams, and best practices. Covers sql, union, union-all development techniques with visual explanations.
Tags:
Categories:

UNION vs. UNION ALL: Understanding the Key Differences in SQL

UNION vs. UNION ALL: Understanding the Key Differences in SQL

Explore the fundamental distinctions between UNION and UNION ALL operators in SQL, including their performance implications and use cases for combining result sets.

In SQL, both UNION and UNION ALL are used to combine the result sets of two or more SELECT statements into a single result set. While they appear similar, a critical difference lies in how they handle duplicate rows. Understanding this distinction is crucial for writing efficient and accurate SQL queries. This article will delve into the mechanics of each operator, illustrate their usage with practical examples, and discuss performance considerations.

The UNION Operator: Eliminating Duplicates

The UNION operator combines the result sets of two or more SELECT statements and removes all duplicate rows from the final result. This means if a row exists in both SELECT statements or multiple times within a single SELECT statement's result set, it will only appear once in the UNION's output. The database engine performs an implicit DISTINCT operation after combining the rows, which can be resource-intensive, especially with large datasets.

SELECT column1, column2 FROM TableA
UNION
SELECT column1, column2 FROM TableB;

Basic syntax for the UNION operator.

A Venn diagram illustrating the UNION operation. Two overlapping circles, labeled 'Result Set A' and 'Result Set B', represent the outputs of two SELECT statements. The shaded area covers both circles, including the overlap, indicating that UNION returns all unique rows from both sets, effectively showing the combined set without duplicates.

Visual representation of the UNION operation.

The UNION ALL Operator: Preserving Duplicates

Conversely, the UNION ALL operator combines the result sets of two or more SELECT statements and retains all duplicate rows. It simply appends the result sets together, without any implicit DISTINCT operation. This makes UNION ALL generally faster than UNION because the database does not need to perform the additional work of sorting and comparing rows to identify and remove duplicates. It's ideal when you know your combined sets won't have duplicates, or when you explicitly want to see all occurrences of rows.

SELECT column1, column2 FROM TableA
UNION ALL
SELECT column1, column2 FROM TableB;

Basic syntax for the UNION ALL operator.

A diagram illustrating the UNION ALL operation. Two separate rectangles, labeled 'Result Set A' and 'Result Set B', are stacked vertically, representing the outputs of two SELECT statements. A larger rectangle below them, labeled 'Combined Result', shows all rows from Result Set A followed by all rows from Result Set B, including any duplicates. No overlap or deduplication is shown, emphasizing the appending nature.

Visual representation of the UNION ALL operation.

Key Differences and Performance Considerations

The primary difference between UNION and UNION ALL lies in their handling of duplicate rows. UNION removes duplicates, while UNION ALL includes them. This distinction has significant implications for query performance and resource usage.

Because UNION performs an implicit DISTINCT operation, it typically requires more processing power and time, especially for large datasets. The database must sort the combined result set to identify and remove duplicate rows. This can involve writing temporary data to disk if the result set is too large to fit in memory. UNION ALL, on the other hand, is a more straightforward operation. It simply concatenates the result sets, making it much faster and less resource-intensive when duplicate removal is not necessary.

When to Use Which Operator

Choosing between UNION and UNION ALL depends entirely on your specific requirements:

  • Use UNION when:

    • You need a distinct list of all values from multiple result sets.
    • Duplicate rows are undesirable in the final output.
    • The performance overhead of duplicate removal is acceptable for your use case.

  • Use UNION ALL when:

    • You need to combine all rows from multiple result sets, including duplicates.
    • Performance is critical, and duplicate removal is not required.
    • You are certain that the individual SELECT statements will not produce duplicates across their combined result set, or duplicates are expected and desired (e.g., combining sales records from different regions, where a customer might appear in both but you want to count all transactions).

Both UNION and UNION ALL require that the SELECT statements have the same number of columns, and the corresponding columns must have compatible data types. If these conditions are not met, the query will result in an error.