In all the transformations that we have created so far, we had single streams of data. We could, however, create more than one stream, with data coming from different sources. The streams can eventually be merged together—as was the case when we merged data coming from different files in the previous chapters—or they can also be used for looking up data, as we will learn in this section.
Looking for data in a secondary stream is a common requirement when the data you need comes from a source that is different from your main data—for example, if your data comes from a database, and you need to look up related data in an XML file. In this section, you will learn how to implement this kind of lookup through a simple exercise: We will have a list of European cities, and we will look for their cost of living indexes that are located in a different source. To do this, go through the following steps:
