UniqueIdentifier are either explicitly provided or they are generated by the NEWID() system function. : NewID « Constraints « SQL Server / T-SQL Tutorial

1. SQL Server / T-SQL Tutorial
2. Constraints
3. NewID

7> CREATE TABLE MyTable (MyID UniqueIdentifier NOT NULL DEFAULT NewID()
8>           , MyDescription nVarChar(50) NOT NULL)
9>
10> INSERT MyTable (MyID, MyDescription) VALUES (DEFAULT, 'SQL Server generated GUID')
11>
12> select * from MyTable;
13> drop table MyTable;
14> GO

(1 rows affected)
MyID                                 MyDescription
------------------------------------ --------------------------------------------------
230A2164-ACD7-4E86-82CA-C92C28A4030C SQL Server generated GUID

(1 rows affected)
1>