Union vs. Union All

Attribute	Union	Union All
Duplicates	Eliminates duplicates	Retains duplicates
Order	Orders the result set	Does not order the result set
Performance	May have slower performance due to duplicate elimination	May have faster performance as it does not eliminate duplicates
Usage	Used when you want to combine and eliminate duplicate rows from multiple result sets	Used when you want to combine multiple result sets without eliminating duplicates

Introduction

When working with databases, it is common to encounter situations where you need to combine data from multiple tables or queries. In SQL, the UNION and UNION ALL operators are used to achieve this. While both operators serve a similar purpose, they have distinct differences that can impact their performance and the results they produce. In this article, we will explore the attributes of UNION and UNION ALL, highlighting their similarities and differences.

UNION Operator

The UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. It eliminates duplicate rows from the final result set, ensuring that each row is unique. The syntax for using UNION is as follows:

SELECT column1, column2, ...  FROM table1  UNION  SELECT column1, column2, ...  FROM table2;

When using UNION, the column names and data types of the corresponding columns in the SELECT statements must match. If they don't, you may need to use explicit type conversions or aliases to ensure compatibility.

UNION ALL Operator

The UNION ALL operator, on the other hand, also combines the result sets of multiple SELECT statements into a single result set. However, unlike UNION, it does not eliminate duplicate rows. The syntax for using UNION ALL is similar to UNION:

SELECT column1, column2, ...  FROM table1  UNION ALL  SELECT column1, column2, ...  FROM table2;

Since UNION ALL does not perform the additional step of removing duplicates, it generally has better performance compared to UNION. However, it is important to note that UNION ALL may return duplicate rows in the final result set, which may or may not be desirable depending on the specific use case.

Performance Considerations

When deciding between UNION and UNION ALL, it is crucial to consider the performance implications. UNION performs an additional step of removing duplicate rows, which can be computationally expensive, especially when dealing with large result sets. On the other hand, UNION ALL simply concatenates the result sets without any duplicate elimination, resulting in faster execution times.

However, if you are certain that the result sets from your SELECT statements do not contain any duplicates or if you want to include duplicate rows in the final result set, using UNION ALL can be a more efficient choice. It avoids the overhead of duplicate elimination and can significantly improve query performance.

Result Set Differences

One of the key differences between UNION and UNION ALL is the way they handle duplicate rows. As mentioned earlier, UNION eliminates duplicate rows from the final result set, ensuring that each row is unique. This can be useful when you want to combine data from multiple sources and remove any duplicate entries.

On the other hand, UNION ALL does not remove duplicate rows. It simply concatenates the result sets, including all rows from each SELECT statement. This can be beneficial when you explicitly want to include duplicate rows in the final result set, or when you are confident that the result sets do not contain any duplicates.

Memory Usage

Another important consideration when using UNION and UNION ALL is the memory usage. Since UNION eliminates duplicate rows, it needs to store the intermediate result set in memory to perform the duplicate elimination. This can consume significant memory resources, especially when dealing with large result sets.

On the other hand, UNION ALL does not require any additional memory for duplicate elimination. It simply concatenates the result sets, resulting in lower memory usage compared to UNION. This can be advantageous when memory resources are limited or when dealing with large datasets.

Conclusion

In summary, the UNION and UNION ALL operators are powerful tools for combining data from multiple tables or queries in SQL. While both operators serve a similar purpose, they have distinct differences that should be considered when choosing between them.

UNION eliminates duplicate rows from the final result set, ensuring uniqueness, but it comes with the cost of additional computational overhead and memory usage. On the other hand, UNION ALL simply concatenates the result sets without removing duplicates, resulting in faster execution times and lower memory usage.

Ultimately, the choice between UNION and UNION ALL depends on the specific requirements of your query and the characteristics of your data. Understanding the attributes and performance implications of each operator will help you make an informed decision and optimize your SQL queries accordingly.