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.