NEWSEQUENTIALID (Transact-SQL)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
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. When a GUID column is used as a row identifier, using NEWSEQUENTIALID can be faster than using the NEWID function. This is because the NEWID function causes random activity and uses fewer cached data pages. Using NEWSEQUENTIALID also helps to completely fill the data and index pages.
Important
If privacy is a concern, do not use this function. It is possible to guess the value of the next generated GUID and, therefore, access data associated with that GUID.
NEWSEQUENTIALID is a wrapper over the Windows UuidCreateSequential function, with some byte shuffling applied.
Warning
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.
Transact-SQL syntax conventions
Syntax
NEWSEQUENTIALID ( )
Return Type
uniqueidentifier
Remarks
NEWSEQUENTIALID() can only be used with DEFAULT constraints on table columns of type uniqueidentifier. For example:
CREATE TABLE myTable (ColumnA uniqueidentifier DEFAULT NEWSEQUENTIALID());
When NEWSEQUENTIALID() is used in DEFAULT expressions, it cannot be combined with other scalar operators. For example, you cannot execute the following:
CREATE TABLE myTable (ColumnA uniqueidentifier DEFAULT dbo.myfunction(NEWSEQUENTIALID()));
In the previous example, myfunction() is a scalar user-defined scalar function that accepts and returns a uniqueidentifier value.
NEWSEQUENTIALID cannot be referenced in queries.
You can use NEWSEQUENTIALID to generate GUIDs to reduce page splits and random IO at the leaf level of indexes.
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.