Is a 16-byte GUID.
A column or local variable of uniqueidentifier data type can be initialized to a value in the following ways:
By using the NEWID function.
By converting from a string constant in the form xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx, in which each x is a hexadecimal digit in the range 0-9 or a-f. For example, 6F9619FF-8B86-D011-B42D-00C04FC964FF is a valid uniqueidentifier value.
Comparison operators can be used with uniqueidentifier values. However, ordering is not implemented by comparing the bit patterns of the two values. The only operations that can be performed against a uniqueidentifier value are comparisons (=, <>, <, >, <=, >=) and checking for NULL (IS NULL and IS NOT NULL). No other arithmetic operators can be used. All column constraints and properties, except IDENTITY, can be used on the uniqueidentifier data type.
Merge replication and transactional replication with updating subscriptions use uniqueidentifier columns to guarantee that rows are uniquely identified across multiple copies of the table.