24
Mon, Jun
3 New Articles

SQL 101: The Simplicity and Flexibility of the SELECT Statement

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

It's time to talk about the Data Manipulation Language (DML). Don't worry; it's not a new programming languagejust a fancy name to a subset of SQL instructions. I'll start with the SELECT statement.

 

Instead of starting by explaining how to build an SQL database from the ground up, let's start by discussing the things that you can use to query and change data in your DDS-defined physical files: a set of SQL instructions used to manipulate data, commonly referred to as the Data Manipulation Language, or DML. Entire books have been written about the DML; my objective here is to provide a helicopter view of the main DML instructions, accompanied by examples whenever possible.

 

Let's visit (or for some readers, re-visit) one of the most-used SQL instructions: the SELECT.

 

The Simplicity and Flexibility of the SELECT Statement

This is arguably the most-used SQL instruction. It can be used, in its simplest form, to retrieve all the records from a table with just four "words": SELECT * FROM <Table>. Naturally, this is not enough for most situations, so let's break down the SELECT statement and explain the different options at our disposal. From here on, I'll use a simplified inventory master file, named InvMst and described in the table below, to provide examples of the different scenarios.

 

The InvMst File Description

Column Name

Data Type

Length

Column Description

ItemID

Character

15,0

Item ID

LotNbr

Decimal

13,0

Lot Number

ExpDate

Date

N/A

Expiration Date

WHID

Decimal

8,0

Warehouse ID

ShelfID

Decimal

12,0

Shelf ID

ItemUn

Character

3

Item Units

ItemQty

Decimal

9,2

Item Quantity

 

This is a simplification of an actual inventory master file, so you'll see that a lot of columns are missing. That's not very important for the examples. What's important is to understand the file's structure: the first five columns provide the record's unique key; ItemID, LotNbr, WHID, and ShelfID are keys to other tables. In SQL lingo, they are this table's foreign keys. Because this is an inventory master file, this table has millions of rows and doing a simple SELECT is neither efficient nor useful.

 

Therefore, our simple four-word SELECT statement is basically useless here. To extract meaningful information from this table, I'll need to "ask" specific questions via a SELECT statement; let me show you how. The basic SELECT structure is the following:

 

SELECT <column 1>

       [, <column 2>, <column 3>…., <column n>

FROM   <table name>

[WHERE <condition 1> <logical operator> <condition 2>, etc]

 

The mandatory parts of a SELECT statement are the SELECT and FROM reserved words, the n column names or expressions (I'll explain this later), and a table name. The asterisk (*) you saw in the basic four-word statement means "all the columns of the table." Optionally, you can specify one or more conditions by using the WHERE reserved word to indicate that and by using logical operators, such as AND and"OR, just like you would do in an RPG IF statement. There are other optional components. If I wanted to know the quantity of item id 'A123' and its respective expiration date available in warehouse 333, I would write the following SELECT statement:

 

SELECT       ItemID

, ItemQty

             , ExpDate

FROM         InvMst

WHERE        ItemID = 'A123'

AND WHID = 333

 

Note that SQL is case-insensitive. I wrote the reserved words in caps just to remind you of the SELECT's structure. It's important to mention that the line breaks and indentations are purely optional; you can write the whole statement in one line, with blank spaces separating the different components. However, line breaks and indentation help improve readability, so it's a good idea to use them!

 

Having said that, let's analyze the statement: I'm retrieving the contents of three columns (ItemId, ItemQty, and ExpDate) from the InvMst table records that match the conditions specified after the WHERE reserved word. But there's more I can do with SELECT: for instance, if I wanted to sort the results by expiration date, I would specify the ORDER BY keyword, followed by the column name. The statement would be the same, with the addition of the ORDER BY keyword and the ExpDate column after it:

 

SELECT       ItemID

, ItemQty

             , ExpDate

FROM         InvMst

WHERE  ItemID = 'A123'

AND WHID = 333

ORDER BY ExpDate

 

Just like in the list of columns I specified after the SELECT, I can include multiple column names in the ORDER BY clause or I can indicate the position of the column instead. In this example, I could have written ORDER BY 3 because ExpDate is the third column in the SELECT. If I wanted the sort to occur in descending order, I would have used the DESC reserved word after ExpDate to indicate that.

 

The list of "things" in the SELECT clause doesn't necessarily have to consist of columns: you can use literal constants and expressions. These expressions can be nearly anything, from a simple "1 + 2" math operation to logical operations (1 = 0 returns a false result), SQL built-in functions, and even your functions, called Stored Procedures or User-Defined Functions (UDFs), depending on how they work, in SQL lingo.

Let's do a quick rundown of the operators at your disposal:

 

Arithmetic operatorsThe addition (+), subtraction (-), multiplication (*), and division (/) operators that you are used to from RPG are the same in SQL.

 

Comparison operators (or predicates)The usual greater than (>), less than (<), equal to (=), and not equal to (<>) operators are available; they keep the same functionality and combinations they have in RPG, such as >=, for instance, but you have additional operators available:

  • The BETWEEN operator is used to replace a field >= value1 and field <= value2 expression with the simpler field BETWEEN value1 AND value2 expression; if you add the NOT operator before this BETWEEN expression, you'll be making it equivalent to the field < value1 or field > value2 expression.
  • The LIKE operator searches for strings that have a certain pattern. If the pattern is found, a true value is returned. Here's a simple example:

SELECT ItemID

FROM         InvMst

WHERE        ItemID like 'A%'

This will select all item IDs that start with 'A', regardless of what follows the 'A' character. The underscore sign (_) represents any single character. The percent sign (%) represents a string of zero or more characters.

  • The IN operator is very useful to replace those field = value1 or field = value2 … or field = valueN expressions with a much more elegant field in (value1, value2, … valueN) expression. You can also prefix this operator with NOT, thus negating the expression.

This was probably nothing new for some readers, but an introduction to the SELECT statement was necessary to lay the foundation for more-complex queries. The next article will explain how you can use more than one table in a SELECT statement and the different ways to join those tables.

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

$0.00 Raised:
$

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: