uniqueidentifier

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.

The GUIDs generated by NEWSEQUENTIALID() are unique only within a particular computer if the computer does not have a network card.

You can use NEWSEQUENTIALID() to generate GUIDs to reduce page contention at the leaf level of indexes.

Reference

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