Monday, July 28, 2008

Selecting Every Other Row

One of the fun things about publishing is getting questions from readers that make you think. A recent question I received went something like this: "Can I get the odd and even number of rows from a DB2 table?"

Well, my first reaction was to think "this guy doesn't understand the way a SQL DBMS like DB2 works." The data in DB2 tables is not ordered, so there is no way to guarantee that the rows are odd or even numbered. While that observation may (or may not) have been true, it didn't help the guy. So I thought about it and came up with a possible work-around solution.

The first thing we have to do is to mimic row numbers in DB2. Until V9, DB2 did not support the row number construct (such as you can find in Oracle), and we'd like this to work for the versions in support today (V8 and V9).

So, to do this we start by using the COUNT(*) function and a table expression. A table expression is when you substitute SQL in place of the table in the FROM clause of another SQL statement. For example, consider this SQL:

SELECT  DEPTNO, ROWNUM
FROM DSN8810.DEPT A,
TABLE (SELECT COUNT(*) + 1 AS ROWNUM
FROM DSN8810.DEPT B
WHERE B.DEPTNO < A.DEPTNO) AS TEMP_TAB;

That puts a pseudo-row number on the table that we can access in our SQL predicates. If, say, we only want to return the even results, we could write the following query:

SELECT  DEPTNO, ROWNUM
FROM DSN8810.DEPT A,
TABLE (SELECT COUNT(*) + 1 AS ROWNUM
FROM DSN8810.DEPT B
WHERE B.DEPTNO < A.DEPTNO) AS TEMP_TAB
WHERE MOD(ROWNUM,2) = 0
ORDER BY ROWNUM;

The MOD function returns the remainder of dividing the second argument into the first. So, if the remainder is zero, we have an even number. So, this query returns every other row to the result set. If you want the odd rows only, change the predicate with the MOD function to this:

WHERE MOD(ROWNUM,2) <> 0

Of course, there is no guarantee that the same exact rows will be even (or odd) for subsequent executions of this query. It all depends how DB2 optimizes the query for execution. But it does provide a nice way to produce samples of the data (perhaps to populate a test bed of data).

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Thursday, July 24, 2008

Free Webinar - Database Auditing for DB2 z/OS - July 29, 2008

Protecting corporate data is a requirement of doing business in today's regulatory and security-minded business environment. Protecting corporate data -- an especially sensitive data -- is a matter of knowing who is accessing data and what are they doing with it. There have been many solutions for addressing this need on distributed databases, but no reasonable solution for protecting mainframe data until now.

Learn all about an exciting new solution for auditing your DB2 for z/OS databases and resources - Guardium for Mainframes - at this free webinar on July 29, 2008.

Guardium for Mainframes provides 100% visibility into mainframe database activities without impacting normal business operations. This webinar will show you how to get better insight into database activity without the performance penalty of typical database trace utilities and without relying on inadequate log file data.

I'll be introducing the webinar and giving a quick overview of the issues, and Bill Baker, a senior software consultant with NEON Enterprise Software, will walk through a demonstration of the Guardium for Mainframes in action!


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Monday, July 21, 2008

New Data Sharing RedPaper

Just a quick FYI today to let you know about a new RedPaper offering information about exploiting client load balancing and fail over capabilities across a DB2 data sharing group (or a subset of the group members).

A RedPaper is sort of like a tip, only longer... and sort of like a RedBook, only shorter... Anyway, if you are interested in the topic, the RedPaper can be donwloaded for free by following this link:

DB2 9 for z/OS Data Sharing: Distributed Load Balancing and Fault Tolerant Configuration

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Monday, July 07, 2008

A Video Interview on Long-term Retention

When I spoke at the Techxans event in Houston this past May (2008) I was interviewed beforehand on what my presentation would cover. And lo' and behold, the Techxans folks have put that interview up on YouTube, so I thought I'd share it here with my regular blog readers. Enjoy!

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