Thu, Jul
2 New Articles

SQL 101: Ultra-Flexible Cursors—The Beauty of PREPARE

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

SQL’s PREPARE statement, combined with ILE RPG, can bring unmatched flexibility to your application’s code. Read on to find how you can do it. It’s easier than you think!

Using thePREPARE SQL instruction, you can have a single cursor and use it according to conditions defined by user input, for instance. I’ve seen subfiles with different columns, selections, and sorting being managed with a single cursor. How, you might ask? The PREPARESQL instruction allows you to define a cursor loosely:


   Exec SQL

     DECLARE mainCursor CURSOR


MY_SELECT is, or rather, will be the SELECT statement that the cursor will use, but at declaration time, MY_SELECT is only a placeholder. You need to define its “variable” content with a PREPARE statement before the DECLARE statement, but you’re free to compose it in any way that you want, as long as you specify a valid SELECT statement. Here’s how you do it:


   W_My_SQL_Stmt = ‘SELECT * FROM InvMst WHERE WHID = ‘ +

                   %CHAR(K_WHID) + ‘ AND ExpDate < CurDate’;

   Exec SQL


These two statements will assign a dynamic statement to the W_My_SQL_Stmt variable. Note that it can be even more dynamic; I could change the columns, the sort, or any other part of the SELECT statement. The resulting string is then transformed into an SQL “thing” that I’ll use in the DECLARE statement to define my cursor. After this operation, the usual OPEN, FETCH, CLOSE cursor procedure is used.

Here’s a practical example: Imagine you want a function that returns the total number of records of a given table or table join. The function is abstract, in the sense that the statement describing the table or table join is not known beforehand and neither are the conditions (the WHERE part of the statement) in which it’ll be run. This is the perfect example of PREPARE’s flexibility. As usual, let’s start with the function’s prototype interface:


* Procedure: RowCounter - return a row count from a select statement*


PRowCounter       B                   EXPORT

DRowCounter       PI                 Like(W_Out)

* Input Parms

DP_Stm                       1000A   Value

* Work variables

DW_Out           S             12 0

Yes, I know, it should be a /FREE definition. It’s not because I’m reusing an actual piece of production code that hasn’t been converted to free-format.

Anyway, there’s nothing really new here; just notice that the input parameter is a regular string. This string will hold a complete (and preferably valid) SELECT statement with an aggregation function in the SELECT clause. Although this is designed to use a SELECT COUNT(*) statement, you can use it with other aggregation functions as well, such as SUM, for instance.

Note that the string’s size (1,000 characters) should be enough for most SQL statements, but if you plan to use complex (or simply very long) SELECT statements, it might not be enough. If you get strange “invalid SQL statement” errors at runtime, you should look into this variable’s size and compare it with the value you’re trying to pass it. There’s only one work variable, W_Out, which will be used to return the output of the SELECT statement, as you’ll see in a minute:



       PREPARE S1 FROM :P_Stm;




       OPEN C1;


       FETCH C1 INTO :W_Out;


       CLOSE C1;

This is the main body of the function. This is an RPG function, but there’s only SQL in it. Let’s analyze the code. The input parameter, P_Stm, is used to create an “SQL thing” named S1 that is going to be used by the C1 cursor’s DECLARE statement as its SELECT part. From here, the code follows the usual cursor operation sequence: OPEN, FETCH, and CLOSE. All that is left to do is return W_Out to the outside world and end the function:

     Return W_Out;


PRowCounter       E

Note that the function doesn’t know (and doesn’t care) what the SELECT statement looks like, as long as it’s a valid SELECT statement that returns a number—the typical result of an aggregation function. PREPARE allows you to create this type of “hollow function” with dynamic SELECT statements, but it can also be used for other dynamic statements, as you’ll see later.

This is a simple example, but I believe it gets the point across. With the PREPARE statement, you can write generic RPG functions that can be reused in several different situations within the same type of scenario (for instance, quickly returning the maximum value that fulfills certain very specific conditions for a variety of different files/variations within the same file) or even fully dynamic Open Query File replacements, as mentioned in the previous TechTip.

That’s all the time we have this time around. In the next article, I’ll be discussing how to embed other SQL statements in RPG…and why you should do it.

Rafael Victoria-Pereira

Rafael Victória-Pereira has more than 20 years of IBM i experience as a programmer, analyst, and manager. Over that period, he has been an active voice in the IBM i community, encouraging and helping programmers transition to ILE and free-format RPG. Rafael has written more than 100 technical articles about topics ranging from interfaces (the topic for his first book, Flexible Input, Dazzling Output with IBM i) to modern RPG and SQL in his popular RPG Academy and SQL 101 series on mcpressonline.com and in his books Evolve Your RPG Coding and SQL for IBM i: A Database Modernization Guide. Rafael writes in an easy-to-read, practical style that is highly popular with his audience of IBM technology professionals.

Rafael is the Deputy IT Director - Infrastructures and Services at the Luis Simões Group in Portugal. His areas of expertise include programming in the IBM i native languages (RPG, CL, and DB2 SQL) and in "modern" programming languages, such as Java, C#, and Python, as well as project management and consultancy.

MC Press books written by Rafael Victória-Pereira available now on the MC Press Bookstore.

Evolve Your RPG Coding: Move from OPM to ILE...and Beyond Evolve Your RPG Coding: Move from OPM to ILE...and Beyond
Transition to modern RPG programming with this step-by-step guide through ILE and free-format RPG, SQL, and modernization techniques.
List Price $79.95

Now On Sale

Flexible Input, Dazzling Output with IBM i Flexible Input, Dazzling Output with IBM i
Uncover easier, more flexible ways to get data into your system, plus some methods for exporting and presenting the vital business data it contains.
List Price $79.95

Now On Sale

SQL for IBM i: A Database Modernization Guide SQL for IBM i: A Database Modernization Guide
Learn how to use SQL’s capabilities to modernize and enhance your IBM i database.
List Price $79.95

Now On Sale



Support MC Press Online


Book Reviews

Resource Center

  • SB Profound WC 5536 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. You can find Part 1 here. In Part 2 of our free Node.js Webinar Series, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Brian will briefly discuss the different tools available, and demonstrate his preferred setup for Node development on IBM i or any platform. Attend this webinar to learn:

  • 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 company are not aligned with the current IT environment.

  • SB HelpSystems ROBOT Generic IBM announced the E1080 servers using the latest Power10 processor in September 2021. The most powerful processor from IBM to date, Power10 is designed to handle the demands of doing business in today’s high-tech atmosphere, including running cloud applications, supporting big data, and managing AI workloads. But what does Power10 mean for your data center? In this recorded webinar, IBMers Dan Sundt and Dylan Boday join IBM Power Champion Tom Huntington for a discussion on why Power10 technology is the right strategic investment if you run IBM i, AIX, or Linux. In this action-packed hour, Tom will share trends from the IBM i and AIX user communities while Dan and Dylan dive into the tech specs for key hardware, including:

  • Magic MarkTRY 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. Make sure your data survives when catastrophe hits. Request your trial now!  Request Now.

  • SB HelpSystems ROBOT GenericForms of ransomware has been around for over 30 years, and with more and more organizations suffering attacks each year, it continues to endure. What has made ransomware such a durable threat and what is the best way to combat it? In order to prevent ransomware, organizations must first understand how it works.

  • SB HelpSystems ROBOT GenericIT security is a top priority for businesses around the world, but most IBM i pros don’t know where to begin—and most cybersecurity experts don’t know IBM i. In this session, Robin Tatam explores the business impact of lax IBM i security, the top vulnerabilities putting IBM i at risk, and the steps you can take to protect your organization. If you’re looking to avoid unexpected downtime or corrupted data, you don’t want to miss this session.

  • SB HelpSystems ROBOT GenericCan you trust all of your users all of the time? A typical end user receives 16 malicious emails each month, but only 17 percent of these phishing campaigns are reported to IT. Once an attack is underway, most organizations won’t discover the breach until six months later. A staggering amount of damage can occur in that time. Despite these risks, 93 percent of organizations are leaving their IBM i systems vulnerable to cybercrime. In this on-demand webinar, IBM i security experts Robin Tatam and Sandi Moore will reveal:

  • FORTRA Disaster protection is vital to every business. Yet, it often consists of patched together procedures that are prone to error. From automatic backups to data encryption to media management, Robot automates the routine (yet often complex) 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:

  • FORTRAManaging messages on your IBM i can be more than a full-time job if you have to do it manually. Messages need a response and resources must be monitored—often over multiple systems and across platforms. 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:

  • FORTRAThe thought of printing, distributing, and storing iSeries reports manually may reduce you to tears. Paper and labor costs associated with report generation can spiral out of control. Mountains of paper threaten to swamp your files. Robot 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:

  • FORTRAFor over 30 years, Robot has been a leader in systems management for IBM i. With batch job creation and scheduling at its core, the Robot Job Scheduling Solution reduces the opportunity for human error and helps you maintain service levels, automating even the biggest, most complex runbooks. Manage your job schedule with the Robot Job Scheduling Solution. Key features include:

  • LANSA Business 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.

  • 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:

  • LANSASupply Chain is becoming increasingly complex and unpredictable. From raw materials for manufacturing to food supply chains, the journey from source to production to delivery to consumers is marred with inefficiencies, manual processes, shortages, recalls, counterfeits, and scandals. In this webinar, we discuss how:

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • Profound Logic 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.

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? 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: