When working with SQL Server, you may come across the UNION and UNION ALL operators. These operators are used to combine the results of two or more SELECT statements into a single result set. However, it is important to understand the differences between UNION and UNION ALL, as they serve different purposes and can have an impact on performance.

UNION Operator

The UNION operator is used to select related information from two or more tables, similar to the JOIN command. However, there are a few key differences. When using the UNION operator, all selected columns need to be of the same data type. Additionally, the UNION operator only selects distinct values, meaning that duplicate rows are eliminated from the result set.

For example, let’s say we have two tables: Customers and Suppliers. We want to retrieve a list of all unique names from both tables. We can achieve this using the UNION operator:

SELECT Name FROM Customers
UNION
SELECT Name FROM Suppliers;

This query will return a result set containing the unique names from both tables.

UNION ALL Operator

The UNION ALL operator, on the other hand, selects all values from the SELECT statements, including duplicates. This means that if there are duplicate rows in the result set, they will not be eliminated.

Using the same example as before, if we want to retrieve a list of all names from both the Customers and Suppliers tables, including duplicates, we can use the UNION ALL operator:

SELECT Name FROM Customers
UNION ALL
SELECT Name FROM Suppliers;

This query will return a result set containing all names from both tables, including any duplicates.

Performance Considerations

When it comes to performance, there is a difference between UNION and UNION ALL. Since the UNION operator eliminates duplicate rows, it may require additional processing time and resources. On the other hand, the UNION ALL operator does not perform this elimination, making it potentially faster.

However, it is important to note that the performance difference between UNION and UNION ALL may vary depending on the specific scenario and the amount of data being processed. It is recommended to test and analyze the performance of your queries to determine which operator is more suitable for your needs.

Conclusion

In summary, the UNION and UNION ALL operators in SQL Server serve different purposes. The UNION operator is used to select distinct values from multiple tables, while the UNION ALL operator selects all values, including duplicates. Understanding the differences between these operators and considering their impact on performance can help you write more efficient and optimized SQL queries.