Sidebar

Practical SQL: Updating a File Using a Cursor

SQL
Typography
  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

SQL isn't just for queries; this article shows you one way to use its update capabilities.

I like to use SQL primarily as a query tool. Its very name—Structured Query Language—sort of leans you in that direction. It's also an excellent tool for set-based updates, as its many proponents will tell you. But there are circumstances under which it would be good to be able to update a file one record at a time via SQL. This is how you do it.

Future-Proofing Your Code

The initial impetus for the concepts in this article came from an ongoing attempt to future-proof the development in a production shop. This shop, like so many, works on a combination of modern programming techniques, including free-format ILE RPG and what can charitably be called long-in-the-tooth packaged code. The goal is to be able to make changes in this code the most productive way possible.

While we've developed processes that allow us to implement changes rapidly, we still run up against one of the most difficult issues in all of RPG development: the dreaded database change. This is a package with thousands of programs, many of which share several major master files. Item master, anyone, or customer order? Transaction history, perhaps? Well, in order to handle database changes in packaged software, you really have only two options: modify the existing database or add extension files. Yes, there are a few other more-involved options, such as converting all existing code to SQL access. Those sorts of architectural overhauls might make sense in a software development shop, but in most production environments, those approaches aren't easy to get approved by the folks who pay the bills. So instead you'll have to choose between modifying the existing files or creating your own extension files.

Modifying Existing Files

Modifying the existing files is difficult, especially in a living, breathing production shop. You can't change the file during normal business operations because every program that has the file open has a lock on it. And it's bad enough in a simple 9-to-5 environment; the complexity simply increases as you throw in multiple shifts and international locations. Pretty soon you get to a spot where you have to set aside a specific window, say two hours in the wee hours on Sunday morning, when the system is unavailable to users. You may be able to schedule that once a week or, in even more complicated cases, once a month. Because of this, a database change requires someone to work in the off hours, installing not only the database changes but also recompiled versions of every affected program. And woe be to you if the database change fails in any way; backing it out is every bit as resource-sensitive, and it's usually an emergency requiring kicking people off during production. This isn't a scenario I recommend.

Creating Extension Files

Enter the extension file. In case you haven’t created one before, an extension file is simply a custom file that you build with the same unique keys as an existing master file. You put all your custom data points in that file. Then, programs that need those data points simply retrieve the appropriate record from the extension file.

When implemented using traditional record-level access (RLA), the extension file suffers from most of the same problems as a normal database file. If you define the file using a traditional F-spec (or a free-format dcl-f statement), the program will lock the file upon startup and hold it until you exit. You can circumvent the lock by using the USROPN keyword on the file definition and opening and closing the file as needed, but even in that scenario, if you change the file, the program will get a level-check error the next time it tries to open the file.

SQL Unbound

SQL provides a real benefit here because of its unbound nature. By that, I mean that SQL works with a more dynamic view of the data than native I/O. The names of the fields are specified in the SQL statement and passed to the SQL engine, which then accesses the data in the file dynamically. If the layout of the table changes, the SQL engine recognizes those changes and processes the new structure accordingly. As long as the changes don't conflict with the basic syntax of the SQL statement, there is no problem.

Now, most people when they think of using SQL to update data think of the traditional SQL UPDATE statement:

      exec sql update ORDEXT

     set OXSTAT = '40' where OXSTAT = '10';

This is a perfectly acceptable way of updating data. Set-based updates are in fact one of the features that SQL evangelists like to point to when they stress the benefits of SQL. And while I agree that updating large numbers of records with a single statement has a certain appeal, it's rare that I have the opportunity to do that. Instead, I typically have to slog through a whole raft of records, testing each case and processing only those that meet certain criteria. Since that selection process often involves multiple files and a lot of conditional logic, it's not very well suited to SQL's version of conditional logic, the adequate but somewhat verbose CASE statement.

That's where the cursor comes in. In RPG's embedded SQL, you can declare a cursor (if you've written an SQLRPGLE program you've almost certainly been down this path). You specify the SELECT criteria and the columns you want to retrieve, and then you loop through the cursor one row at a time using the FETCH NEXT statement.

That's for a read-only cursor. But what if you want to update the data? Well, that only requires one little extra clause on the DECLARE statement and then suddenly you can now update the data as well. Take a look at the following code.

      exec sql declare Orders cursor for

     select OXORDR from ORDEXT where OXSTAT = '10'

        order by OXCUST, OXORDR

        for update of OXSTAT;

     exec sql open Orders;

      dow (1=1);

     exec sql fetch next from Orders into :wOrder;

     if SQLCOD <> *zeros;

        leave;

     endif;

     if Process(wOrder);

        exec sql update ORDEXT

             set OXSTAT = '40' where current of Orders;

     endif;

      enddo;

     exec sql close Orders;

Here I've defined a cursor named Orders, and all that cursor does is retrieve the order number from the extension file. Most of the statement is vanilla SQL syntax, including a WHERE clause and an ORDER BY clause. Note that I use the same condition (OXSTAT = '10') as the original UPDATE clause. But it isn't until the very end that things get a little unusual. The FOR UPDATE clause indicates to the SQL engine not only that the program intends to update the file, but also which field (or fields) we expect it to update.

Next, I use one of my standard programming constructs: an infinite loop using the FETCH NEXT to get the next record; I exit the loop on EOF. But here's where we see a little extra logic. I execute a custom subprocedure called Process, which does some additional processing. If Process is successful, it will return *ON, which will in turn direct the program to execute the UPDATE statement.

And here's the tiny detail that makes all the difference. The UPDATE statement doesn't use a traditional WHERE clause that updates every record in the file in which some field or fields match the selection criteria. Instead, the Process routine adds additional checks, and only if those are passed does it return *ON. But the beauty of this technique is that it will work no matter how much you change the file (with obvious exceptions).

Note that this holds as long as you don't use the dreaded "SELECT *" syntax against the physical table. In another article, I'll spend a little more time on the pros and cons of using data structures and views for your SQL access, but that will have to wait. For now, take your time and see if an updatable cursor might just solve one of your business needs.

Joe Pluta

Joe Pluta is the founder and chief architect of Pluta Brothers Design, Inc. He has been extending the IBM midrange since the days of the IBM System/3. Joe uses WebSphere extensively, especially as the base for PSC/400, the only product that can move your legacy systems to the Web using simple green-screen commands. He has written several books, including Developing Web 2.0 Applications with EGL for IBM i, E-Deployment: The Fastest Path to the Web, Eclipse: Step by Step, and WDSC: Step by Step. Joe performs onsite mentoring and speaks at user groups around the country. You can reach him at joepluta@plutabrothers.com.


MC Press books written by Joe Pluta available now on the MC Press Bookstore.

Developing Web 2.0 Applications with EGL for IBM i Developing Web 2.0 Applications with EGL for IBM i
Joe Pluta introduces you to EGL Rich UI and IBM’s Rational Developer for the IBM i platform.
List Price $39.95

Now On Sale

WDSC: Step by Step WDSC: Step by Step
Discover incredibly powerful WDSC with this easy-to-understand yet thorough introduction.
List Price $74.95

Now On Sale

Eclipse: Step by Step Eclipse: Step by Step
Quickly get up to speed and productivity using Eclipse.
List Price $59.00

Now On Sale

More Articles By This Author
Related Articles
BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

RESOURCE CENTER

  • WHITE PAPERS

  • WEBCAST

  • TRIAL SOFTWARE

  • White Paper: Node.js for Enterprise IBM i Modernization

    SB Profound WP 5539

    If your business is thinking about modernizing your legacy IBM i (also known as AS/400 or iSeries) applications, you will want to read this white paper first!

    Download this paper and learn how Node.js can ensure that you:
    - Modernize on-time and budget - no more lengthy, costly, disruptive app rewrites!
    - Retain your IBM i systems of record
    - Find and hire new development talent
    - Integrate new Node.js applications with your existing RPG, Java, .Net, and PHP apps
    - Extend your IBM i capabilties to include Watson API, Cloud, and Internet of Things


    Read Node.js for Enterprise IBM i Modernization Now!

     

  • Profound Logic Solution Guide

    SB Profound WP 5539More than ever, there is a demand for IT to deliver innovation.
    Your IBM i has been an essential part of your business operations for years. However, your organization may struggle to maintain the current system and implement new projects.
    The thousands of customers we've worked with and surveyed state that expectations regarding the digital footprint and vision of the companyare not aligned with the current IT environment.

    Get your copy of this important guide today!

     

  • 2022 IBM i Marketplace Survey Results

    Fortra2022 marks the eighth edition of the IBM i Marketplace Survey Results. Each year, Fortra captures data on how businesses use the IBM i platform and the IT and cybersecurity initiatives it supports.

    Over the years, this survey has become a true industry benchmark, revealing to readers the trends that are shaping and driving the market and providing insight into what the future may bring for this technology.

  • Brunswick bowls a perfect 300 with LANSA!

    FortraBrunswick is the leader in bowling products, services, and industry expertise for the development and renovation of new and existing bowling centers and mixed-use recreation facilities across the entertainment industry. However, the lifeblood of Brunswick’s capital equipment business was running on a 15-year-old software application written in Visual Basic 6 (VB6) with a SQL Server back-end. The application was at the end of its life and needed to be replaced.
    With the help of Visual LANSA, they found an easy-to-use, long-term platform that enabled their team to collaborate, innovate, and integrate with existing systems and databases within a single platform.
    Read the case study to learn how they achieved success and increased the speed of development by 30% with Visual LANSA.

     

  • Progressive Web Apps: Create a Universal Experience Across All Devices

    LANSAProgressive Web Apps allow you to reach anyone, anywhere, and on any device with a single unified codebase. This means that your applications—regardless of browser, device, or platform—instantly become more reliable and consistent. They are the present and future of application development, and more and more businesses are catching on.
    Download this whitepaper and learn:

    • How PWAs support fast application development and streamline DevOps
    • How to give your business a competitive edge using PWAs
    • What makes progressive web apps so versatile, both online and offline

     

     

  • The Power of Coding in a Low-Code Solution

    LANSAWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed.
    Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

    • Discover the benefits of Low-code's quick application creation
    • Understand the differences in model-based and language-based Low-Code platforms
    • Explore the strengths of LANSA's Low-Code Solution to Low-Code’s biggest drawbacks

     

     

  • Why Migrate When You Can Modernize?

    LANSABusiness users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.
    In this white paper, you’ll learn how to think of these issues as opportunities rather than problems. We’ll explore motivations to migrate or modernize, their risks and considerations you should be aware of before embarking on a (migration or modernization) project.
    Lastly, we’ll discuss how modernizing IBM i applications with optimized business workflows, integration with other technologies and new mobile and web user interfaces will enable IT – and the business – to experience time-added value and much more.

     

  • UPDATED: Developer Kit: Making a Business Case for Modernization and Beyond

    Profound Logic Software, Inc.Having trouble getting management approval for modernization projects? The problem may be you're not speaking enough "business" to them.

    This Developer Kit provides you study-backed data and a ready-to-use business case template to help get your very next development project approved!

  • What to Do When Your AS/400 Talent Retires

    FortraIT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators is small.

    This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn:

    • Why IBM i skills depletion is a top concern
    • How leading organizations are coping
    • Where automation will make the biggest impact

     

  • Node.js on IBM i Webinar Series Pt. 2: Setting Up Your Development Tools

    Profound Logic Software, Inc.Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application. In Part 2, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Attend this webinar to learn:

    • Different tools to develop Node.js applications on IBM i
    • Debugging Node.js
    • The basics of Git and tools to help those new to it
    • Using NodeRun.com as a pre-built development environment

     

     

  • Expert Tips for IBM i Security: Beyond the Basics

    SB PowerTech WC GenericIn this session, IBM i security expert Robin Tatam provides a quick recap of IBM i security basics and guides you through some advanced cybersecurity techniques that can help you take data protection to the next level. Robin will cover:

    • Reducing the risk posed by special authorities
    • Establishing object-level security
    • Overseeing user actions and data access

    Don't miss this chance to take your knowledge of IBM i security beyond the basics.

     

     

  • 5 IBM i Security Quick Wins

    SB PowerTech WC GenericIn today’s threat landscape, upper management is laser-focused on cybersecurity. You need to make progress in securing your systems—and make it fast.
    There’s no shortage of actions you could take, but what tactics will actually deliver the results you need? And how can you find a security strategy that fits your budget and time constraints?
    Join top IBM i security expert Robin Tatam as he outlines the five fastest and most impactful changes you can make to strengthen IBM i security this year.
    Your system didn’t become unsecure overnight and you won’t be able to turn it around overnight either. But quick wins are possible with IBM i security, and Robin Tatam will show you how to achieve them.

  • Security Bulletin: Malware Infection Discovered on IBM i Server!

    SB PowerTech WC GenericMalicious programs can bring entire businesses to their knees—and IBM i shops are not immune. It’s critical to grasp the true impact malware can have on IBM i and the network that connects to it. Attend this webinar to gain a thorough understanding of the relationships between:

    • Viruses, native objects, and the integrated file system (IFS)
    • Power Systems and Windows-based viruses and malware
    • PC-based anti-virus scanning versus native IBM i scanning

    There are a number of ways you can minimize your exposure to viruses. IBM i security expert Sandi Moore explains the facts, including how to ensure you're fully protected and compliant with regulations such as PCI.

     

     

  • Encryption on IBM i Simplified

    SB PowerTech WC GenericDB2 Field Procedures (FieldProcs) were introduced in IBM i 7.1 and have greatly simplified encryption, often without requiring any application changes. Now you can quickly encrypt sensitive data on the IBM i including PII, PCI, PHI data in your physical files and tables.
    Watch this webinar to learn how you can quickly implement encryption on the IBM i. During the webinar, security expert Robin Tatam will show you how to:

    • Use Field Procedures to automate encryption and decryption
    • Restrict and mask field level access by user or group
    • Meet compliance requirements with effective key management and audit trails

     

  • Lessons Learned from IBM i Cyber Attacks

    SB PowerTech WC GenericDespite the many options IBM has provided to protect your systems and data, many organizations still struggle to apply appropriate security controls.
    In this webinar, you'll get insight into how the criminals accessed these systems, the fallout from these attacks, and how the incidents could have been avoided by following security best practices.

    • Learn which security gaps cyber criminals love most
    • Find out how other IBM i organizations have fallen victim
    • Get the details on policies and processes you can implement to protect your organization, even when staff works from home

    You will learn the steps you can take to avoid the mistakes made in these examples, as well as other inadequate and misconfigured settings that put businesses at risk.

     

     

  • The Power of Coding in a Low-Code Solution

    SB PowerTech WC GenericWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed.
    Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

    • Discover the benefits of Low-code's quick application creation
    • Understand the differences in model-based and language-based Low-Code platforms
    • Explore the strengths of LANSA's Low-Code Solution to Low-Code’s biggest drawbacks

     

     

  • Node Webinar Series Pt. 1: The World of Node.js on IBM i

    SB Profound WC GenericHave you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application.
    Part 1 will teach you what Node.js is, why it's a great option for IBM i shops, and how to take advantage of the ecosystem surrounding Node.
    In addition to background information, our Director of Product Development Scott Klement will demonstrate applications that take advantage of the Node Package Manager (npm).
    Watch Now.

  • The Biggest Mistakes in IBM i Security

    SB Profound WC Generic The Biggest Mistakes in IBM i Security
    Here’s the harsh reality: cybersecurity pros have to get their jobs right every single day, while an attacker only has to succeed once to do incredible damage.
    Whether that’s thousands of exposed records, millions of dollars in fines and legal fees, or diminished share value, it’s easy to judge organizations that fall victim. IBM i enjoys an enviable reputation for security, but no system is impervious to mistakes.
    Join this webinar to learn about the biggest errors made when securing a Power Systems server.
    This knowledge is critical for ensuring integrity of your application data and preventing you from becoming the next Equifax. It’s also essential for complying with all formal regulations, including SOX, PCI, GDPR, and HIPAA
    Watch Now.

  • Comply in 5! Well, actually UNDER 5 minutes!!

    SB CYBRA PPL 5382

    TRY the one package that solves all your document design and printing challenges on all your platforms.

    Produce bar code labels, electronic forms, ad hoc reports, and RFID tags – without programming! MarkMagic is the only document design and print solution that combines report writing, WYSIWYG label and forms design, and conditional printing in one integrated product.

    Request your trial now!

  • Backup and Recovery on IBM i: Your Strategy for the Unexpected

    FortraRobot automates the routine tasks of iSeries backup and recovery, saving you time and money and making the process safer and more reliable. Automate your backups with the Robot Backup and Recovery Solution. Key features include:
    - Simplified backup procedures
    - Easy data encryption
    - Save media management
    - Guided restoration
    - Seamless product integration
    Make sure your data survives when catastrophe hits. Try the Robot Backup and Recovery Solution FREE for 30 days.

  • Manage IBM i Messages by Exception with Robot

    SB HelpSystems SC 5413Managing messages on your IBM i can be more than a full-time job if you have to do it manually. How can you be sure you won’t miss important system events?
    Automate your message center with the Robot Message Management Solution. Key features include:
    - Automated message management
    - Tailored notifications and automatic escalation
    - System-wide control of your IBM i partitions
    - Two-way system notifications from your mobile device
    - Seamless product integration
    Try the Robot Message Management Solution FREE for 30 days.

  • Easiest Way to Save Money? Stop Printing IBM i Reports

    FortraRobot automates report bursting, distribution, bundling, and archiving, and offers secure, selective online report viewing.
    Manage your reports with the Robot Report Management Solution. Key features include:

    - Automated report distribution
    - View online without delay
    - Browser interface to make notes
    - Custom retention capabilities
    - Seamless product integration
    Rerun another report? Never again. Try the Robot Report Management Solution FREE for 30 days.

  • Hassle-Free IBM i Operations around the Clock

    SB HelpSystems SC 5413For over 30 years, Robot has been a leader in systems management for IBM i.
    Manage your job schedule with the Robot Job Scheduling Solution. Key features include:
    - Automated batch, interactive, and cross-platform scheduling
    - Event-driven dependency processing
    - Centralized monitoring and reporting
    - Audit log and ready-to-use reports
    - Seamless product integration
    Scale your software, not your staff. Try the Robot Job Scheduling Solution FREE for 30 days.