SQL Server Documentation

uniqueidentifier Data Type

The uniqueidentifier data type stores a 16-byte globally unique identifier (GUID). A GUID is a unique number that is used to identify a process, an application, or a system. GUIDs are generated by the operating system.

Description

The uniqueidentifier data type is useful for primary keys, and for storing transaction IDs. You can use the NEWID() function to generate a new GUID.

Syntax

DECLARE @myGuid uniqueidentifier
SET @myGuid = NEWID()
PRINT @myGuid

Example Usage

Here is an example of how to create a table with a uniqueidentifier column and insert a new GUID into it:

CREATE TABLE Products (
    ProductID uniqueidentifier PRIMARY KEY DEFAULT NEWID(),
    ProductName varchar(100) NOT NULL
);

INSERT INTO Products (ProductName)
VALUES ('Awesome Widget');

SELECT * FROM Products;

GUID Format

GUIDs are typically represented as a string of 32 hexadecimal digits, displayed in five groups separated by hyphens, in the format 8-4-4-4-12. For example: A1B2C3D4-E5F6-7890-1234-567890ABCDEF.

Key Functions

• NEWID(): Generates a new unique identifier (GUID).
• NEWSEQUENTIALID(): Generates a GUID that is larger than any previously generated GUID on the computer. This function can be useful for clustered primary keys.

Considerations

• When using uniqueidentifier as a primary key, consider the performance implications of random GUIDs versus sequential GUIDs, especially with large tables and clustered indexes. NEWSEQUENTIALID() can help mitigate fragmentation issues.
• Ensure that your application or client libraries correctly handle the conversion between the string representation of a GUID and the 16-byte binary format.

Related Topics

• NEWID() Function
• NEWSEQUENTIALID() Function
• CREATE TABLE
• Primary Keys