Why Is There No Default Order in SQL Queries Without ORDER BY?

The Default Order of SQL Queries

The default order of a SQL query when no ORDER BY clause is present has been a puzzling question for many. Unlike other programming languages, SQL does not provide an inherent order for data retrieval.

What Happens When ORDER BY is Omitted?

Contrary to common assumptions, there is no implicit ordering when the ORDER BY clause is absent. The ordering of rows may vary depending on internal factors, the type of storage engine used, and potential optimizations applied by the database.

The documentation for the default sort order of SELECT queries without an ORDER BY clause further cautions against relying on a specific ordering:

1. Do not assume any order when the ORDER BY clause is missing.
2. Always specify ORDER BY if a particular order is desired. The optimizer may remove the clause if it conflicts with other operations.
3. GROUP BY implicitly adds an ORDER BY clause. However, this behavior is non-standard and can be avoided by using ORDER BY NULL.

Implications for Table Scans

In certain situations, such as table scans, the order of returned rows may initially appear to follow the order of insertion. However, this is a mere artifact of the underlying implementation and should not be relied upon. In fact, different storage engines (e.g., InnoDB) may produce differing ordering results.

Conclusion

The default order of SQL queries without an ORDER BY clause is essentially undefined. To ensure consistent and predictable ordering, it is crucial to explicitly specify the desired order using the ORDER BY clause.

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