Does UNION ALL Preserve Result Order?

Maintaining Result Order with UNION ALL: A Closer Look

In the realm of SQL, the UNION ALL operator combines multiple result sets while retaining duplicate rows. However, this operator does not inherently guarantee the order of the resulting data. This article explores whether it's possible to ensure the result set always follows a specific order and provides a solution to achieve this.

Does UNION ALL Guarantee Order?

The answer is a resounding no. Without explicit instructions, UNION ALL will not order the resulting data in a predetermined sequence. The order of the rows in the final result depends on the order in which the individual SELECT statements are executed, which can vary depending on factors such as server workload and database optimization techniques.

Proof of Order Variation

Consider the following example:

SELECT 'O';
UNION ALL
SELECT 'R';
UNION ALL
SELECT 'D';
UNION ALL
SELECT 'E';
UNION ALL
SELECT 'R';

There is no way to determine the order of the result set in this scenario. The rows could be returned in any order, such as 'E-R-O-D-R' or 'O-E-D-R-R'.

Ensuring Order with ORDER BY

To maintain a specific order in the result set, you must explicitly specify the desired sequence using the ORDER BY clause. For instance, in our example, we can add a SortOrder column to each SELECT statement, ensuring the records appear in the desired order:

SELECT 'O', 1 AS SortOrder;
UNION ALL
SELECT 'R', 2;
UNION ALL
SELECT 'D', 3;
UNION ALL
SELECT 'E', 4;
UNION ALL
SELECT 'R', 5;
ORDER BY SortOrder;

With this modification, the result set will always be sorted according to the SortOrder column, guaranteeing that the order is always 'O-R-D-E-R'.

Conclusion

UNION ALL does not guarantee the order of the result set. However, by explicitly specifying the desired order using ORDER BY, you can ensure that the rows are returned in the correct sequence. This simple yet effective technique is essential for ensuring data consistency and meeting specific ordering requirements.

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