If you are upgrading from SharePoint 2007 to 2010 there are some different ways to upgrade, but one of the most useful if you are not doing an in place upgrade is to use the stsadm command with addcontentdb.
stsadm –o addcontentdb –url http://newsite–databasename 2007Content
or you can use the PowerShell command Mount-SPContentDatabase. Essentially they do the same thing but since we are moving toward PowerShell this is good opportunity to get some practice in. In both cases, you are upgrading an individual content database.
NOTE: If you try to restore the content database (normally you do this on a backup of the content database) twice to the same farm it will fail because SharePoint doesn’t allow two site collections with the same GUID.
Before you upgrade the content database you will want to run the PowerShell command Test-SPContentDatabase . You run this from the SharePoint 2010 server and point it toward SharePoint 2007 content databases. This will give you a lot of useful information about what is missing from the web application you are upgrading to. Most of this is going to be features and solutions that are missing. Most of the time you can still upgrade with these errors and install the features/solutions after you do the upgrade. Or you may want to remove those features. It is a best practice to clean up these items before you do the upgrade.
Points to remember.
Always upgrade a backup of the content database (or have a current backup handy) because even if it fails, it is a one way trip. If it does fail, you are left with a pretty useless content database.
Allow for some space on your SQL Server. During the upgrade process you need some room on the hard drive. I would recommend at least half as much space as the size of the content database.
Allow for some time. This is a fairly slow process that depends on a lot of factors. The number of site collections seems to be one of the bigger ones. This is a process that could take hours.
Make sure your server has enough memory. If you are skimpy on the memory this process could fail.
Only for normal content databases and not things like the old SSP.
Don’t try to do this with the same content twice to the same SharePoint farm (i.e. you tried it once as a test and then tried again for real). You will need to remove the upgraded content database from the web app before you can try again (or delete the web application of course).
Good luck with your upgrade. I hope it goes well!!