ANSI SQL is extremely limited when it comes to date formatting, but DB2 provides a powerful extension that alleviates that problem.
SQL is becoming ever more popular in most IBM i shops, and for good reason: it makes it very easy to do things that are perhaps not so easy in RPG. SQL also provides a standard interface to various tools that aren't specific to the platform, primarily through ODBC and JDBC. However, SQL isn't a silver bullet. There are things that are easier and faster in RPG, and some things that are nearly impossible to do in SQL.
For the longest time, date formatting has been one of the biggest shortcomings of SQL. For whatever reason, ANSI SQL still has no standard function for formatting dates. In most cases, you end up with something along the lines of this:
SUBSTR(DIGITS(ORDDAT,5,2)) || '/' ||
SUBSTR(DIGITS(ORDDAT,7,2)) || '/' ||
SUBSTR(DIGITS(ORDDAT,3,2))) AS DATMDY
And this is a fairly easy case, where the date is already formatted into a CCYYMMDD field. Not particularly intuitive and definitely a whole lot of typing, which in turn means a whole lot of opportunity for typos. That propensity for error increases as you add more date fields.
What Are the Alternatives?
One way to address the problem is to write your own user-defined function (UDF). I actually did that by extending Alan Campin's excellent iDate function, available from Think400. The standard iDate utility converts non-date data into variables of the actual SQL date type. This is the converse of today's topic, but it's definitely a tool you're going to want to have in your arsenal. I just added some routines written in RPG to convert date variables to numbers (for legacy databases) and strings (for reports).
The other option is to rely on the vendor-specific extensions, if any. Microsoft SQL Server, for example, provides a very specific conversion function not surprisingly named CONVERT. This function takes a date and returns a variable of the selected type. For example:
CONVERT (VARCHAR, myDate, 101)
This returns the date in mm/dd/yyyy format. CONVERT can also be used to convert a string into a date variable using the same style values. There are about 20 different style values, and about a dozen of those have two-digit year variants.
The problem, of course, is that this function isn't available on DB2. In fact, prior to DB2 8.5 or so, you had exactly one possibility: you could convert a timestamp to YYYY-DD-MM HH(24):MI:SS using the function VARCHAR_FORMAT. That's the only supported format. If you went to the InfoCenter documentation on the function you saw a section that said "Valid format strings are:" and then that one format string. Click here if you think I'm kidding.
This is what we had on the IBM i up until V5R4. We had no other options (although I did manage to get it to work with YYYY-MM-DD). However, in DB2 9.5 (and in DB2 for the IBM i v6.1), IBM added a whole raft of other conversion options. Check out the InfoCenter page for more information, but generally speaking just about any conversion you can think of is now available. This includes standard values such as two-digit months and hours in either 12- or 24-hour format, advanced values such as either full or partial month names (as an example, although the page doesn't show it, MON in either upper, lower, or title case will return the abbreviated month name in the corresponding case), and even obscure options such as ISO or standard week number, or quarter (1-4).
So now you can get rid of your strange and twisted substring notations and come into the wonderful world of real date formats:
SELECT ITEMNO, TO_CHAR(LASTMNT, 'DD-Mon-YYYY') FROM ITMMST
Notice that I used TO_CHAR instead of VARCHAR_FORMAT. TO_CHAR is a great synonym for those of us who are typing challenged. Also, TO_CHAR requires real date-type dates, but if your database isn't yet upgraded to all date data types, you can still use iDate to convert them for you. And that's a great discussion for another day. Have fun!