Practical SQL: Complex Filters Using Common Table Expressions

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

SQL allows you to do many things, but encapsulating logic isn't one of them…unless you understand the magic of Common Table Expressions (CTEs).

Recently, I've been doing a lot of data analysis. A lot of data analysis. I'm reviewing legacy data on a system that's been in production for over 20 years, and I’m trying to do forensic analysis on it. One of my projects is implementing some enhanced logic, but before it can be put into service, the stakeholders have to approve the changes, and the only way that happens is if I can provide detailed before and after numbers for the auditors. That means finding a way to apply ad hoc complex filters quickly and effectively in order to separate high- and low-impact effects.

They Be Stealin' My Bucket!

Sorry for the gratuitous use of an Internet meme; it happens to be one of my son's favorites. If you want to find it, just Google LOLRUS. But I digress; I just wanted a fun way to bring the word bucket into play, because that's the focus of this article. One of the most common filtering tasks I have to do during this analysis is to break up the data I'm summarizing into buckets by date. This wouldn't be too difficult except that the buckets are somewhat arbitrary, and they also change based on the subset of the data I'm looking at. Older data can be broken down by years, while newer data has to be down to months or even weeks. And the really tricky bit is that I don't know what the buckets need to be until I start breaking the data down into buckets.

And here's the best part: this is legacy data, which means the dates are stored as decimal values, not true dates. And we all know just how easy it is to perform date arithmetic with decimal numbers in SQL, right? Let's take a peek at the sort of calculation I have to do and see if you recognize the issue as one you've run into yourself. Let's say that, for the purposes of this example, I need to summarize production quantities into buckets. The calculations are a bit complex, including production quantities and values, but in order to keep the example manageable, I'll use only a single data point, the production quantity. So basically, I need to go through the file PRODHIST and summarize the quantity produced (PHQTY). Summarization needs to be by manufacturing site (PHSITE) as well as into buckets by date.

The date field is receipt date, PHRCDT, an eight-digit field. I'm going to give you the first, simplest breakdown: by one year, two years, and more than two years. We'll run off of July 1 of this year. Here's the first pass:

select PHSITE,

   sum(case when PHRCDT < 20150701

       then PHQTY else 0 end) YR2PLUS,

   sum(case when PHRCDT >= 20150701 and PHRCDT < 20160701

       then PHQTY else 0 end) YR2,

   sum(case when PHRCDT >= 20160701 and PHRCDT < 20170701

       then PHQTY else 0 end) YR1,

   sum(case when PHRCDT >= 20170701

       then PHQTY else 0 end) FUTURE

from PRODHIST group by PHSITE

What this will do is sum the PHQTY by PHSITE into four buckets.

Site       YR2PLUS            YR2            YR1          FUTURE

EAST   10,100,330.45   1,554,612.75   1,653,212.70     140,214.21


It's pretty simple. And it's relatively easy when you have hardcoded cutoffs. But what if you need to change the bucketing, perhaps to something a little more granular? As an example, let's look at the FUTURE bucket. We included it only on a whim because we know there can be no receipts later than today, but remember this is legacy data and you never know what you might find. So now the auditors would like all future data broken down by months, something like this:


   sum(case when PHRCDT >= 20170701 and PHRCDT < 20170801

       then PHQTY else 0 end) MO1,

   sum(case when PHRCDT >= 20150801 and PHRCDT < 20160901

       then PHQTY else 0 end) MO2,

   sum(case when PHRCDT >= 20150901 and PHRCDT < 20161001

       then PHQTY else 0 end) MO3,

   sum(case when PHRCDT >= 20171001

       then PHQTY else 0 end) MO4PLUS


This wasn't terribly hard, because the number of columns was the same and again, I had hardcoded dates. All that were needed were a few modifications to the existing buckets and an extra comparison on the first one to skip older data. Once again, pretty simple. But now let's do a thought exercise. Say the auditors love this, but they want to run it regularly based on the current date. Ah, now we get interesting.

Date Arithmetic in SQL

This shouldn't be too hard, right? Basically, I just want something like this:

sum(case when PHRCDT >= CURRENT_DATE

         and PHRCDT < CURRENT_DATE + 1 MONTH

   then PHQTY else 0 end) MO1,

I have to repeat this for each bucket, but I think that would be acceptable if it weren't for the pesky fact that PHRCDT isn't a date field. It's a legacy standard eight-digit decimal field, and that's not going to work very well. Now, I could update the statement to dynamically convert one of the operands or the other so that they are compatible. Back in the olden days that might be something like this:

sum (case

when date(substr(digits(PHRCDT),1,4) || '-' ||    

           substr(digits(PHRCDT),5,2) || '-' ||    

           substr(digits(PHRCDT),7,2)) >= CURRENT_DATE

   and date(substr(digits(PHRCDT),1,4) || '-' ||    

           substr(digits(PHRCDT),5,2) || '-' ||    

           substr(digits(PHRCDT),7,2)) < CURRENT_DATE + 1 MONTH

then PHQTY else 0 end) MO1,

Just imagine that repeated for every bucket. Suddenly a simple bucketing explodes into dozens of lines of code that are easy to break with a simple transposition. I've actually seen code like this, and I'll wager you have, too. The other option is to convert the date to CCYYMMDD, something I talked about in a previous article on date formatting. It works, but it isn't much cleaner:


when PHRCDT >=

   decimal(to_char(CURRENT_TIMESTAMP, 'YYYYMMDD'),8,0))

and PHRCDT <

   decimal(to_char(CURRENT_TIMESTAMP + 1 MONTH, 'YYYYMMDD'),8,0))

then PHQTY else 0 end) MO1,

There are other way to convert dates to decimal, but they're no easier to understand, and I still need to repeat whatever I do for every bucket. Twice, in fact. And if the auditors need me to summarize another field (say, the count of receipts), I then have to duplicate that whole mess again. The readability is way down, the chance for a typo way up, and all in all this is just going in a bad direction.

Common Table Expressions to the Rescue!

And now to the solution, our good friend the Common Table Expression, or CTE! With the CTE, I can do all the bucketing calculations up front and then use those results for my buckets, no matter how many I need. I can easily change the bucketing in one place.

First, let's look at the modified code.

-- Define bucket cutoff dates

with t1(LOWEST, B1CUT, B2CUT, B3CUT) as (values (              


   decimal(to_char(CURRENT_TIMESTAMP + 1 MONTHS,'YYYYMMDD'),8,0),

   decimal(to_char(CURRENT_TIMESTAMP + 2 MONTHS,'YYYYMMDD'),8,0),

   decimal(to_char(CURRENT_TIMESTAMP + 3 MONTHS,'YYYYMMDD'),8,0)))          

-- Summarize production into buckets


   sum(case when PHRCDT >= LOWEST and PHRCDT < B1CUT

       then PHQTY else 0 end) BUCKET1,

   sum(case when PHRCDT >= B1CUT and PHRCDT < B2CUT

       then PHQTY else 0 end) YR2,

   sum(case when PHRCDT >= B2CUT and PHRCDT < B3CUT

       then PHQTY else 0 end) YR1,

   sum(case when PHRCDT >= B3CUT

       then PHQTY else 0 end) BUCKET4

from PRODHIST cross join T1 group by PHSITE, PHCLAS

The definition of the CTE T1 allows us to create as many buckets as we need and assign names to all of them. In this case, the lowest summarized date is today's date, and subsequent cutoffs are today plus one month, plus two months, and plus three months. That particular code is in its own section.

In the next, separate section of the SQL statement, I use those values to summarize my data. I've changed the names of the resulting sums to be the more generic BUCKET1–BUCKET4, which would make it pretty easy to extend the code. Also, you might notice the use of the rather rare CROSS JOIN clause. It's rare because it means to join every record in the first table with every record in the second table, which isn't something you normally do. But it makes perfect sense here, because the T1 CTE has only one record, and I want to use that record to act on every record in the PRODHIST file.

So now, whenever the auditors want a slightly different bucketing of the results, I just change the definition of T1. If I'm particularly lazy (I mean efficient!), I just change the second half to some arbitrarily large number of buckets, say 10. If the auditors ask for 5 buckets, I just repeat the last cutoff for buckets 6–10 and don't include those columns in the report I hand back to them. Done!

So, I hope you get a feel for how you can use CTEs to encapsulate complex logic in your SQL statements. This will reduce duplication and make your code less error-prone and more future-proof. Have fun with CTEs!

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

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


Support MC Press Online





  • 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 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.