Understanding SQL Server Unique Identifier

Welcome, Dev! In this article, we will explore the concept of Unique Identifier in SQL Server. Unique Identifier is a data type that is used for storing globally unique identifiers or GUIDs. GUID is a 128-bit integer value consisting of hex numbers and is used for uniquely identifying records in a database. In this article, we will discuss how GUIDs work, how they are generated, and how they are used in SQL Server.

What is a GUID?

A GUID or Globally Unique Identifier is a unique 128-bit integer value that is generated by a computer system. A GUID consists of hex numbers separated by hyphens and looks something like this:

Example GUID
6F9619FF-8B86-D011-B42D-00C04FC964FF

This value is generated in such a way that it is almost impossible for two different GUIDs to be the same, even if generated by different machines or at different times. GUIDs are often used in distributed systems where multiple databases need to be synchronized, and it is imperative that each record has a unique identifier.

How does SQL Server Generate GUIDs?

SQL Server has a built-in function to generate GUIDs called NEWID(). This function generates a new random GUID every time it is called. The syntax for using NEWID() is:

SELECT NEWID()

This will generate a new GUID every time the query is executed. It is worth noting that the GUID generated by SQL Server using NEWID() is not guaranteed to be truly random, but it is unique enough for most practical purposes.

Using Unique Identifier in SQL Server

Unique Identifier is a data type in SQL Server that can be used to store GUIDs. To create a table with a column of data type Unique Identifier, the syntax is:

CREATE TABLE MyTable (Id UNIQUEIDENTIFIER PRIMARY KEY, Name VARCHAR(50))

In the above example, we have created a table called MyTable with a column called Id of type Unique Identifier. We have also specified that this column is the primary key for the table. This means that the value in the Id column must be unique for each record in the table.

Inserting Records into a Table with Unique Identifier Column

When inserting records into a table with a Unique Identifier column, you can either explicitly specify a value for the column or let SQL Server generate a new GUID for you. For example:

INSERT INTO MyTable (Id, Name) VALUES (‘6F9619FF-8B86-D011-B42D-00C04FC964FF’, ‘John Doe’)

In the above example, we have explicitly specified the value for the Id column. Alternatively, we could have used the NEWID() function to generate a new GUID:

INSERT INTO MyTable (Id, Name) VALUES (NEWID(), ‘Jane Doe’)

This will generate a new GUID and insert it along with the Name value into the table.

Retrieving Records from a Table with Unique Identifier Column

When retrieving records from a table with a Unique Identifier column, you can use the SELECT statement:

SELECT * FROM MyTable WHERE Id = ‘6F9619FF-8B86-D011-B42D-00C04FC964FF’

In the above example, we are retrieving all records from the MyTable table where the Id column matches the specified GUID.

FAQ

1. Can I change the value of a Unique Identifier column in SQL Server?

No, once a value has been assigned to a Unique Identifier column, it cannot be changed.

2. Can I use Unique Identifier as a foreign key?

Yes, Unique Identifier can be used as a foreign key in SQL Server. However, it is generally not recommended as it can impact performance due to the large size of the data type.

3. Are GUIDs always unique?

In practice, GUIDs are almost always unique. However, it is theoretically possible for two different GUIDs to be the same, although the probability is extremely low.

4. What is the maximum length of a Unique Identifier?

The maximum length of a Unique Identifier is 36 characters.

5. Can I use Unique Identifier as a clustered index?

Yes, Unique Identifier can be used as a clustered index in SQL Server. However, it is generally not recommended as it can impact performance due to the large size of the data type.