Well, there are two ways to do this. The first, as suggested above, is to compile the data into a single database. The problem with this is the single database is essentially a mirror of all the existing data, and will have to be kept constantly updated if you want real-time searching. The second is to write a query for each database and then just output all the results on one page. Which method you use largely depends on the number of searches vs the number of updates and how time-sensitive the data is. If you have a large number of global searches, the single database will be more efficient. If there's fewer searches and a lot of updates (and the number of databases is under a few dozen), the multiple-query method may work better. It's hard to give advice without knowing a lot more about your data.
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