DB2 AS400: Sequence Numbering With RRN

Adding Sequence Numbers Based on RRN in DB2 for AS400: A Comprehensive Guide

Hey everyone! Today, we're diving into a common challenge faced when working with DB2 on the AS400 (also known as IBM i): how to add a sequence number to a file without a key, especially when you need to base that sequence on the Relative Record Number (RRN). This is super useful for tasks like sorting data when you don't have a natural key or for creating a unique identifier when one doesn't exist. We'll walk through the problem, explore solutions, and provide practical examples to get you sorted. Let's get started, guys!

The Problem: Sequencing Without a Key

So, you've got a file – let's say it's called RECIPES – and it looks something like this:

Notice something? There's no key! This means traditional SQL ORDER BY operations based on a key won't work. You need a way to assign a sequence number to each row, and that's where the RRN comes in handy. The RRN is like a hidden row identifier, the physical location of the record within the file. DB2 keeps track of it, and we can leverage it. Think of it as the address of each recipe ingredient inside the database. Since the file doesn't have an actual key, this is our only means to sort it. It is also the way to differentiate each record as unique, even if the recipe number and ingredient are the same. This is critical if you need to do more complex operations later, such as updates or deletions, as you don't want to accidentally modify or get rid of the wrong record. If we don't assign a sequence number, it would be hard to keep track of the records.

Understanding the RRN

The Relative Record Number (RRN) is a system-assigned number that identifies the physical location of a record within a file. It's a crucial piece of information when dealing with files that lack a defined key. The RRN isn't stored in the file itself, but it's accessible through SQL using the ROW_NUMBER() window function (which we'll cover later) or through other methods specific to the AS400 environment. Using the RRN is great because it will preserve the record sequence when you are pulling the data. This sequence is consistent, unless the file is reorganized or data is deleted. Let me say this again, do not be fooled by using the RRN if records are deleted. The RRN value can be reused by new records.

Solution 1: Using ROW_NUMBER()

For DB2 on AS400, the ROW_NUMBER() window function is your best friend. It allows you to assign a unique sequence number to each row based on a specified order. Here's how you can use it:

SELECT
    RECIPE_NUMBER,
    INGREDIENT_NUMBER,
    INGREDIENT_QTY,
    ROW_NUMBER() OVER (ORDER BY RRN(RECIPES)) AS SEQUENCE_NO
FROM
    RECIPES;

• ROW_NUMBER() OVER (ORDER BY RRN(RECIPES)): This is the heart of the solution. ROW_NUMBER() assigns a unique integer to each row. The OVER (ORDER BY RRN(RECIPES)) part specifies that the sequence should be assigned based on the RRN of the RECIPES file. The RRN() function is the key here; it provides access to the hidden RRN. Note that the RRN is a DB2 function and not a column. The ORDER BY RRN will sort the data based on the hidden RRN.
• RECIPE_NUMBER, INGREDIENT_NUMBER, INGREDIENT_QTY: These are your existing columns. Include them to see all the data.

Important Considerations for using ROW_NUMBER(): This approach is a simple way to view your data. But, the sequence is generated on the fly when you run the SELECT statement. If you want to store this sequence number in your RECIPES file, you'll need to update the file. You can't directly modify the existing table in place using only the ROW_NUMBER() method.

Solution 2: Updating the File with the Sequence Number

If you want to store the sequence number in the RECIPES file, you'll need to add a column to store the sequence number and then populate it. Here's how you do it. First, add a new column to your RECIPES file to store the sequence number. Assuming the original file doesn't have a key:

ALTER TABLE RECIPES
ADD COLUMN SEQUENCE_NO INTEGER;

This adds an integer column named SEQUENCE_NO to your RECIPES file. Next, we'll update the file with the sequence numbers based on the RRN. Second, use an UPDATE statement with ROW_NUMBER():

UPDATE RECIPES
SET SEQUENCE_NO = Q.SEQUENCE_NO
FROM (
    SELECT
        RRN(RECIPES) AS RRN,
        ROW_NUMBER() OVER (ORDER BY RRN(RECIPES)) AS SEQUENCE_NO
    FROM
        RECIPES
) AS Q
WHERE
    RRN(RECIPES) = Q.RRN;

Let's break down this UPDATE statement, it looks a bit complex, but it works:

• Subquery (Q): The subquery is the same as the previous SELECT statement, except we are retrieving the RRN. It generates the sequence numbers based on the RRN.
• RRN(RECIPES) AS RRN: The RRN of the record. Note that we alias the RRN function to use it later.
• ROW_NUMBER() OVER (ORDER BY RRN(RECIPES)) AS SEQUENCE_NO: As before, this assigns the sequence number. The assigned value will be stored in the new column.
• UPDATE RECIPES SET SEQUENCE_NO = Q.SEQUENCE_NO: This is where the magic happens. It updates the SEQUENCE_NO column in the RECIPES file with the values generated by the subquery.
• WHERE RRN(RECIPES) = Q.RRN: This is the crucial part. It ensures that each record is updated with the correct sequence number based on its RRN. Note that this WHERE condition is based on the alias of the RRN column.

After running this UPDATE statement, your RECIPES file will now have the SEQUENCE_NO column populated with the assigned sequence numbers.

Solution 3: Using a Stored Procedure (Advanced)

For more complex scenarios or to encapsulate the logic, you can use a stored procedure. This offers more flexibility and can improve performance, especially for large files. This approach requires a bit more setup, but it is usually more performant and gives more control over the process. Here's an outline of how to do this:

1. Create the Stored Procedure:

   CREATE PROCEDURE UPDATE_RECIPES_SEQUENCE()
   LANGUAGE SQL
   BEGIN
       DECLARE i INTEGER;
       DECLARE rec_rrn INTEGER;
       DECLARE cur CURSOR FOR SELECT RRN(RECIPES) FROM RECIPES ORDER BY RRN(RECIPES);
       SET i = 1;
       OPEN cur;
       FETCH cur INTO rec_rrn;
       WHILE SQLCODE = 0 DO
           UPDATE RECIPES
           SET SEQUENCE_NO = i
           WHERE RRN(RECIPES) = rec_rrn;
           SET i = i + 1;
           FETCH cur INTO rec_rrn;
       END WHILE;
       CLOSE cur;
   END;
   

   • This stored procedure declares an integer variable i to store the sequence number, and rec_rrn to store the RRN. It uses a cursor to loop through the RECIPES file, retrieving the RRN for each record.
   • Inside the loop, it updates the SEQUENCE_NO column with the current value of i. It is very important to use the RRN function as the WHERE condition.

2. Call the Stored Procedure:

   CALL UPDATE_RECIPES_SEQUENCE();
   

   This executes the stored procedure, which updates the sequence numbers in the RECIPES file. The advantage of using a stored procedure is that you can reuse the same code to assign sequence numbers. You can also add error handling inside the procedure, so you can handle errors gracefully.

Important Considerations

• File Locking: Be aware of file locking issues. When updating the file, ensure that other processes aren't accessing it simultaneously to avoid conflicts. You might need to implement locking mechanisms if concurrent access is a concern. Usually, this is not a problem. But, you should be aware of the problem, especially if you are updating a huge table.
• Performance: For very large files, the UPDATE statement can be time-consuming. Consider breaking the update process into smaller batches if performance is a major concern. Using stored procedures might offer some performance advantages, as they can be precompiled and optimized by the database engine.
• Data Integrity: Always back up your data before performing any large-scale updates. If you have any trouble, you can rollback your change. Doing so will prevent data loss.
• RRN Stability: Remember that the RRN is based on the physical position of the records. If the file is reorganized, the RRN values will change. The sequence numbers are stored in the table. So, your data will be consistent unless the data is reloaded. This will change the data. Make sure that you understand the implications of reorganizing your file. Typically, this is a rare event.

Conclusion

Adding sequence numbers based on RRN in DB2 for AS400 is a powerful technique when dealing with files that lack keys. Using ROW_NUMBER(), the UPDATE statement, or a stored procedure gives you the flexibility to generate, and store sequence numbers. By understanding the RRN and how to use it in your SQL queries, you can efficiently manage and manipulate your data. This empowers you to sort, filter, and process your data more effectively. Happy coding, guys! Keep in mind that the best approach depends on your specific requirements and the size of your data. Always test thoroughly before implementing changes in a production environment.