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In the world of databases, the means of generating unique, sequential values can be crucial for various applications. Two such methods of generating these values are DB2 Identity Columns and SQL Server Sequences. This blog post will delve into each of these methods and compare their features, advantages, and limitations using technologies such as SQL Server 2012, SQL Server 2014, MySQL 5.6, DB2 10.5, and Azure SQL.

DB2 Identity Columns provide a straightforward mechanism for generating unique values. The database system automatically generates these values for every new row inserted into a table. DB2 allows you to customize the start of the sequence, the increment, and whether the sequence should wrap around upon reaching the maximum value. Here's a sample DB2 SQL code snippet that demonstrates the creation of a table with an Identity Column:

CREATE TABLE Employees (
    ID INT GENERATED BY DEFAULT AS IDENTITY,
    Name VARCHAR(100),
    Address VARCHAR(100)
)

On the other hand, SQL Server Sequences, introduced in SQL Server 2012, offer a more flexible approach to generating unique values. Unlike Identity Columns, Sequences are objects that are independent of tables, thus allowing for the sharing of sequence values across multiple tables. They also support caching of values for performance improvement, and enable the manual resetting of the sequence. Here's a sample T-SQL code snippet illustrating the creation and usage of a Sequence:

CREATE SEQUENCE EmployeeIDSequence AS INT
START WITH 1
INCREMENT BY 1;


INSERT INTO Employees (ID, Name, Address)
VALUES (NEXT VALUE FOR EmployeeIDSequence, 'John Doe', '123 Elm St')

One of the main differences between DB2 Identity Columns and SQL Server Sequences is in their flexibility and scope. While Identity Columns are tied to a specific table and column, Sequences can be used across multiple tables and columns. This offers greater flexibility, especially in complex database schemas. However, this flexibility can also lead to potential issues with data integrity if not managed properly.

Furthermore, the support for these features varies across different database systems. For instance, MySQL 5.6 does not natively support sequences, but has an AUTO_INCREMENT attribute which functions similarly to an Identity Column. Azure SQL supports both Identity Columns and Sequences, thereby offering the flexibility and power of both techniques.

In conclusion, both DB2 Identity Columns and SQL Server Sequences offer effective mechanisms to generate unique, sequential values. The choice between them depends largely on your specific database requirements and the database system you are using. Understanding the nuances of each technique is crucial for making an informed decision and ensuring optimal database performance.