Qfyilyi

Is NEWSEQUENTIALID unique within a database? We plan many tables in a database with NEWSEQUENTIALID, inserting simultaneously and want to ensure every guid will be unique, regardless if they are in different tables.

Regular NEWID() will guarantee uniqueness in a database, and chances of having duplicate is chance of being struck by asteroid/lightning, however they do not alleviate the fragmentation issue.

This article talks about Mac address, however I want to specify question for database specific above.
https://stackoverflow.com/questions/28404964/is-uniqueidentifier-unique-across-databases

According to Microsoft Docs the value is unique on a per computer basis, and also on multiple computers as long as the computer in question has a network card (assumedly the generation algorithm uses some values taken from the network card to generate the GUID)

Each GUID generated by using NEWSEQUENTIALID is unique on that computer. GUIDs generated by using NEWSEQUENTIALID are unique across multiple computers only if the source computer has a network card.

Other than this though, there do seem to be some caveats around that uniqueness:

The UuidCreateSequential function has hardware dependencies. On SQL Server, clusters of sequential values can develop when databases (such as contained databases) are moved to other computers. When using Always On and on SQL Database, clusters of sequential values can develop if the database fails over to a different computer.

And on the page for UuidCreateSequential we can also see that the function is guaranteed to be unique across computers if the network card is present

Computers with ethernet/token ring addresses generate UUIDs that are guaranteed to be globally unique.

Acording to the documentation from Microsoft.

NEWSEQUENTIALID

Creates a GUID that is greater than any GUID previously generated by
this function on a specified computer since Windows was started. After
restarting Windows, the GUID can start again from a lower range, but
is still globally unique

I recommend using NEWSEQUENTIALID over NEWID() for any case that that you aren't relying on the built in randomness, mainly due to the fragmentation issues associated with random GUIDs. There is a fantastic article by Kimberly Tripp on some of the pitfalls of using NEWID() on a Primary key.

There are a few situations where NEWSEQUENTIALID may not guarantee uniqueness as are listed in the documentation.

The UuidCreateSequential function has hardware dependencies. On SQL
Server, clusters of sequential values can develop when databases (such
as contained databases) are moved to other computers. When using
Always On and on SQL Database, clusters of sequential values can
develop if the database fails over to a different computer.

However judging by the phrasing of your question it seems like this will be contained on a single database server.

It is also worth noting(not that it applies here) that NEWSEQUENTIALID can be predicted so if privacy of that GUID is a concern this could also be a problem.