Ted Holt
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Guru: Conditional SQL Unions
July 17, 2017 Ted Holt
SQL unions combine two or more result sets into one. That’s what they were designed to do. But unions also provide a way to choose between alternate result sets, i.e. to enable or disable SELECT statements at run time. I have used this feature to advantage on numerous occasions. Here’s how it’s done.
First consider the nature of unions. Each result set of a union can return data or no data, depending on the criteria in the WHERE and HAVING clauses. If a SELECT retrieves no data, the system appends an empty set to the union result. The way to …
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Guru: Error Handling in SQL PL, Part 1
June 12, 2017 Ted Holt
I once fancied myself a logical thinker. I changed my mind when I started programming computers. I quickly realized that I was incapable of writing an error-free program. Chalk up another valuable lesson to experience. More experience taught me to program for both expected and unexpected conditions, and now I apply that concept to all languages that I use, including SQL PL.
SQL PL has excellent exception-handling methods, and they’re not hard to use. In this article and Part 2 to follow, we look at how DB2 informs you that your SQL request worked correctly or not. Next, we’ll take …
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Guru: Common Table Expressions Can Replace Query Chains
June 5, 2017 Ted Holt
Modernization efforts often concentrate on the database and programs. That is well and good, but there is more to modernization. Replacing Query for IBM i with more modern query tools is also important. But what do you do with those queries that people depend on? More confusing, what do you do with query chains?
A query chain is a series of queries that run one after another, consolidating and reformatting data in temporary physical files, in order to produce a resulting data set, often in report form. It’s not unusual to see small CL programs like the following one:
PGM
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Guru: How To Cancel A Bad SQL Update
May 15, 2017 Ted Holt
In Three Ways To Manage Unmatched Data I wrote about the use of the RAISE_ERROR function to force a SELECT statement to cancel when unmatched data is considered a fatal error. Another good use of RAISE_ERROR is to force an UPDATE statement to cancel when an invalid condition occurs.
To illustrate, imagine that you and I work in a factory. All factories have inventory. The people we serve purchase some inventory items and manufacture others. Our job is to write a program that will allow certain people to zero out the inventory balance for certain types of purchased items.
The …
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Guru: The Three Ways to Insert
May 8, 2017 Ted Holt
The INSERT statement is THE (as in the only) SQL way to add new data to a relational database table. At the risk of sounding like a GEICO commercial, “Everybody knows that.” Well, did you know that the INSERT statement supports three distinct ways to add new rows to a table?
To illustrate the three forms of INSERT, imagine that you and I work for a small company that stores goods in, and ships goods from, a warehouse. Since the company has only one warehouse, there has never been a need for a warehouse ID column in any of …
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Guru: SQL Table Functions Can Do Non-function Things
May 1, 2017 Ted Holt
We IBM i developers owe a great debt to Scott Forstie. He’s responsible for the wonderful DB2 for i Services and IBM i Services, which give us SQL interfaces for many functions of the operating system. Like IBM, we can write SQL interfaces to help us with non-database tasks. I recently did exactly that.
While I can’t say that I never use the Start SQL Interactive Session (STRSQL) command, I can say that I prefer to use GUI SQL clients, in particular the Run SQL Scripts utility that is part of IBM i Access Client Solutions (ACS). I do get …
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Guru: Three Ways To Manage Unmatched Data
April 24, 2017 Ted Holt
Heaven forbid that I would ignore a failed RPG CHAIN (random read) operation. I always take appropriate action. Which action I take depends on the situation. The same applies to outer joins that don’t find matching data in a secondary table. Here are three ways to deal with unmatched data in an outer join using SQL.
To illustrate, let’s use three tables from an overly simplified general ledger system. The first is a table of departments into which the business is divided. The second is a chart of accounts. The third is a transaction file that feeds the general ledger. …
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Guru: SQL Facts Of UNION And ORDER BY, Take Two
April 17, 2017 Ted Holt
Hey, Ted:
In last week’s tip, you mentioned that expressions are not allowed in the ORDER BY clause of a union. You can use the union as a subquery to allow the use of an expression for the ordering. An extra layer, but it gives you the result you are looking for.
–S. Sims
Sims is correct. I was so focused on the fact that the ORDER BY of a union does not allow expressions that I completely forgot about a workaround. His (her?) technique is probably the most common way to deal with this limitation. I’ve seen numerous examples …
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Guru: SQL Facts Of UNION And ORDER BY
April 10, 2017 Ted Holt
UNION and ORDER BY are powerful SQL features, but put the two together and you may get some strange and frustrating error messages. Fortunately for us DB2 for i professionals, there are easy ways to make the two collaborate and cooperate. Today is a great day to be sure we understand them.
We need some data for examples. For some of the queries, I use a customer master table (file) and a vendor master table. These are a company’s trading partners, so the two have many attributes in common, especially names and addresses. I also use two sales history tables, …
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Guru: Sorting Options For CPYTOIMPF
March 27, 2017 Ted Holt
I hope that whoever came up with the idea for the Copy to Import File (CPYTOIMPF) command was well compensated. When I think of the time and effort that that command has saved me and countless others, I feel deep gratitude. The addition of the ORDERBY parameter increased the usefulness of CPYTOIMPF, and I’d like to share that with you.
CPYTOIMPF copies a single-format database file (table, physical file, view, or logical file) to a stream file or physical file in a format that is acceptable to another system or application. Probably the most common use of this command is …
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