SQL Server UniqueIdentifier Data Type Tutorial

What is UniqueIdentifier?

UniqueIdentifier is a data type in SQL Server that stores a 16-byte binary value representing a GUID.

GUIDs are generated to be unique across space and time, which means no two GUIDs should ever be the same.

• Size: 16 bytes (128 bits).
• Format: Typically displayed as a 36-character string including hyphens.
• Purpose: To provide unique values for keys or identifiers.

Generating GUIDs in SQL Server

SQL Server provides built-in functions to generate GUIDs: NEWID() and NEWSEQUENTIALID().

NEWID() generates a random GUID, while NEWSEQUENTIALID() generates GUIDs in sequential order to improve index performance.

• NEWID() is commonly used for unique values but can cause fragmentation in indexes.
• NEWSEQUENTIALID() reduces fragmentation by generating GUIDs in sequence but can only be used as a default constraint on a column.

Using UniqueIdentifier as Primary Key

UniqueIdentifier can be used as a primary key to ensure uniqueness across distributed databases.

However, due to its size and randomness, it may impact performance and increase index fragmentation.

• GUIDs are larger than integers, increasing storage and index size.
• Random GUIDs can cause page splits and fragmentation in clustered indexes.
• Sequential GUIDs help mitigate fragmentation but have usage constraints.

Best Practices for Using UniqueIdentifier

To optimize the use of UniqueIdentifier, follow best practices that balance uniqueness and performance.

• Use NEWSEQUENTIALID() for primary keys when possible to reduce fragmentation.
• Avoid using UniqueIdentifier as clustered index if performance is critical.
• Consider using integers for keys if uniqueness across systems is not required.
• Use UniqueIdentifier for keys that must be unique across multiple databases or servers.

Example: Creating a Table with UniqueIdentifier

Here is an example of creating a table with a UniqueIdentifier primary key using NEWID() as the default value.