As you’ve probably seen by now, once you create multiple site collections in the same web application, you don’t get a lot of options as far as database location. Well, it’s really not that complicated:

  • Create the databases for the new site collections (easy enough in CA)
  • Run the following PowerShell command for each site collection, or put it in a script if you feel like it:

Move-SPSite <YourSiteURL> –DestinationDatabase <ContentDBName>

That’s it, easy as pie. See more information on TechNet: