29
Fri, Nov
0 New Articles

SQL 101: DML Recap—Joining Tables, Part 1

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

They say no man is an island. We’re social creatures and (most of us) need to live in a community. In a properly structured database, no table is an island either, so we need to know how to join them in our queries.

Listing the contents of a table, even with comparisons (or search conditions) that constrain the output, as shown in the previous TechTip, is a bit limited in a real-life situation. Typically, our queries are built using information from multiple tables. I’ll go through almost all the ways you can join tables in SQL; I’ll leave a special type of join, called EXCEPTION JOIN, for later. Let’s start with the simplest of them all: the INNER JOIN.

The JOIN You’ve Been Using Without Realizing: The INNER JOIN

The INNER JOIN is arguably the most-used type of join. Actually, you’ve probably been using it without realizing it, because it can be hidden in the WHERE clause, in what is commonly called an implicit join.

For instance, let’s say you want to list all the students who have enrolled at least one class. This is a complete intersection between the Students and Classes tables, a classic INNER JOIN (even though it could be simply a SELECT over the Classes table because it contains the student name, but let’s ignore that for the moment, because it will be useful to illustrate another type of join). We know, from two articles ago, that the link between the PFSTM (Students table) and the PFCLM (Classes table) is the student name. It’s true that it’s not a brilliant solution, but we’ll have to live with it…for now. An SQL statement that lists all the students that have, at any given point, enrolled in a class is often written using an implicit join, like this:

SELECT      STNM

            , CLNM

            , CLCN

   FROM     UMADB_CHP2.PFSTM ST, UMADB_CHP2.PFCLM CL

WHERE      CL.CLSN = ST.STNM

;

However, if there are more than two tables in the SELECT statement, things might get a little hazy. That’s why I think it’s much clearer to write the same statement using an INNER JOIN:

SELECT      STNM

            , CLNM

            , CLCN

   FROM     UMADB_CHP2.PFSTM ST

   INNER JOIN     UMADB_CHP2.PFCLM CL ON CL.CLSN = ST.STNM

;

If you’re familiar with the INNER JOIN syntax, there’s really nothing new for you here, except perhaps the use of aliases for the tables (ST and CL), which make the statement slightly more readable. However, if you’re used to the implicit join instead, there are a couple of things worth mentioning. First, notice how the line following the FROM clause starts with the join type: in this case it’s an INNER JOIN, but there are others, as you’ll see in a moment. The type of join identifier is then followed by the table name (and optionally, an alias), which in turn is followed by the ON keyword. This keyword is used to specify how the connection between the tables is supposed to work. In this case, the link between the Students and Classes tables is achieved via the student name, but it could be a more complex condition, resorting to multiple columns.

Let me close this article with a more complete example that uses multiple INNER JOINs to link all the tables of our sample database. The objective is to obtain a bit more information about a student’s grades and the classes he or she attended. Here’s the statement:

SELECT      STNM AS STUDENT_NAME

            , CONM AS COURSE_NAME

            , CODE AS COURSE_DIRECTOR

            , TETR AS TEACHER_RANK

            , CLNM AS CLASS_NAME

            , CLCY AS CLASS_YEAR

            , GRGR AS GRADE

   FROM     UMADB_CHP2.PFSTM ST

   INNER JOIN UMADB_CHP2.PFCLM CL ON      ST.STNM = CL.CLSN

   INNER JOIN UMADB_CHP2.PFGRM GR ON      GR.GRCN = CL.CLNM

                                    AND GR.GRCY= CL.CLCY

                                    AND GR.GRSN= ST.STNM

   INNER JOIN UMADB_CHP2.PFCOM CO ON      CO.CONM = CL.CLCN

   INNER JOIN UMADB_CHP2.PFTEM TE ON      TE.TENM = CO.CODE

   WHERE    GRSN = 'Anthony, Mark'

;

Even though the statement is a bit longer than the previous examples, it’s a simple SELECT. The difference is that it uses a lot more tables, which can make it confusing really quickly. I’d like to emphasize the importance of indentation to improve the statement’s readability and the use of user-friendly column names. Also note that I’ve used aliases for all the tables. At this time, the aliases are not critically important, because the column names all identify the name of the table to which they belong. In a future article, I’ll show you how to “get the best of both worlds” by providing long, human-readable names and keeping the short “RPG-standard” cryptic names that currently exist. Then you’ll see that making a habit of using aliases for the tables and using them when referring to columns in your statements is of paramount importance to the readability and maintainability of your code.

That’s all for now. I hope this gave you something to think about, especially how to apply these tricks to your own code!

 

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

BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

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: