Why do the results of ORDER BY statements in SQL sorting sometimes seem random?

SQL sorting: OrdER BY statements in order uncertainty

In the process of learning SQL, the sorting behavior of ORDER BY clause is sometimes puzzling. This article will explore the uncertainty of the ordering of ORDER BY statements in the "MICK-SQL Basic Tutorial" and explain the reasons behind it.

The tutorial points out that when the sorted field values ​​in ORDER BY clause are the same, the order of the result sets may not be fixed, but rather seemingly random. This is different from the perception that database query results are usually expected to be in order and consistent.

This statement is not wrong. Many database systems do have uncertainty in the final order when dealing with the case where the sorting field values ​​are equal in ORDER BY clause. This is mainly related to the following factors:

• Sort algorithm: Different sorting algorithms may have different output order when processing the same key values.
• Data storage and retrieval mechanism: Data storage and retrieval methods at the underlying database, such as the order in which data is read from disk, will also affect the order of the final results.

If you always get consistent results in actual operations, this is most likely due to the query caching mechanism of the database. When SQL statements and data remain unchanged, the database will return the results directly from the cache, resulting in the same result for each query.

To verify the sort uncertainty of ORDER BY when processing the same value, you can try:

1. Modify the data in the database.
2. Change the query criteria.
3. Re-execute the query.

If the order of results changes, it means that the database is indeed uncertain when processing the same values ​​in ORDER BY clause.

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