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Summary

SQL Server provides a way to select any one row from a group of rows, provided you write the statement using a specific syntax. This method returns any one row from each group, not the minimum, maximum or anything else. In principle, the one row chosen from each group is unpredictable.

There have been problems with the SelSeqPrjToAnyAgg optimizer transformation used.

The table below summarises the current situation:

For details, read the full article on 𝕏.

 

Summary

Update processing is extremely complicated, much more so than is commonly appreciated. SQL Server generally takes great care to ensure correct operation at all times. It does this partly by protecting physical structures with appropriate latches and locks in accordance with the requirements of the current transaction isolation level.