Using XML, you can integrate data from different sources and stage it for Web access using simple protocols such as the Hypertext Transfer Protocol. If analysis is your goal, consider some special-purpose database features. For example, using data mining and online analytical processing to find important patterns in data are high on many users' lists, especially users working with health care and financial data. It is both possible and desirable to integrate historical data for trend analysis with current data for pattern analysis. To maintain this parallel-processing architecture separately from your transaction database, you should quiz vendors about their ability to populate hypercubes automatically.
Some vendors claim their products can support both analysis and ordinary use.
But get proof. In the past, you would have chosen one database, then laboriously ported in everything. In many cases, this would be impossible, either because of the sheer amount of data or the outrageous cost of re-engineering legacy databases. Commercial merger-mania has created a demand for quickly integrating large and incompatible databases. Look for databases that can communicate with other databases, including their competitors.