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Crystal Reports: How to structure your report and subreport

Subreports are an advanced feature that can solve some complex database reporting problems. These include:

Data for the report are coming from more than one data source
Table joins cannot be used to retrieve the required records
Several views of the same data are required

If you have these problems, display some of the report as a main report and the remainder in one or more subreports.

Then you have an important design decision to make. Which part of the report do you do in the main report, and which parts do you want in subreports?

We suggest the following approach:

The main report must do the primary record selection and grouping. You cannot group or select data based on values in the subreport. The subreport is then used complete the missing values–essentially "fill in the blanks."

One extension to this approach is to use a subreport to retrieve a value, and then use conditional formatting in the main report that uses the subreport information. But there are limitations to this:

The subreport has to take up some vertical space in the report.
Subreports in suppressed sections do not get processed, so any shared variables expect from them will not be calculated.

Why is there so much interest in subreports? This concept is not new to Crystal Reports. It's been in practice for a long time, in fact. Subreports are common database tools that serve as intermediaries when you can’t create a report directly from multiple data sources.

In Access, for example, doing anything across multiple queries or tables requires building a subreport. Crystal Reports doesn’t need the same degree of subreport "propping up" that Access does, but it still makes handy use of subreports.

Suppose you have two databases, where each stores the same information—such as employee names, addresses, start dates, and so on. But, the one uses the date format YYYY-MM-DD and the other uses the date format MM-DD-YY.

Hmm. Small problem, here. Suppose one table store numbers in a true number field and the other stores them as string variables or text. The only way to resolve such incompatibilities is with a subreport. You create one subreport to standardize the data in one database to the format of the data in the other. Or, if you’re feeling particularly masochistic, you create a subreport for each database, coming up with an entirely new format.

Subreports can be linked (they update data as those data change in the source) or unlinked (they contain static data). You would want a linked report if you want "real time" information. You would want a static report under any of these conditions:

You simply lack the space for the larger file created by a linked file.
You won’t have access to the source file, thus you cannot keep the files linked.
Your report focuses on a specific period. For example, if you are reporting last year’s sales and it’s June now, you don’t want new data.

Among linked reports, a increasingly popular option is the on-demand subreport. It doesn’t process until you ask it to. It can appear as just a link or some other element (such as a graphic that says, "Update data now.") This has all kinds of benefits which will become apparent as you use it.

A note on linked subreports

Linked subreports are really useful. Link a field or formula from the main report and get the subreport to select records based on that field. But what if your subreport needs the linked parameter for something else? You might want to use it in a calculation, or for conditional grouping, sorting or section suppress.

If you don’t use the link for record selection you may find that the link is removed when you exit the subreport link form.

The solution is to create the parameter in the subreport prior to creating the link. When you create a link to an existing parameter in the subreport, it is saved when you exit the subreport link form.

This article is copyrighted by Crystalkeen, Mindconnection, and Chelsea Technologies Ltd. It may be freely copied and distributed as long as the original copyright is displayed and no modifications are made to this material. Extracts are permitted. The names Crystal Reports and Seagate Info are trademarks owned by Business Objects.