Restore Web Application By Using Content Database Backup

Below are the steps on how to restore a SharePoint web application by using the content database backup. (e.g. Testing2 is the name of content database backup)

1. Go to the SharePoint central administration and create new web application with the same name & database name that you will restore.
2. In Application Management >> Content databases, select your newly created web application and then delete the newly created content db (remove content database)
3. In Sql Server, restore your content database backup. If there's an error saying that it's currently in use, just wait for 3 to 5 mins and then try again.
4. In Application Management >> Content databases, add a content database. Set the database name equal to the restored content db in step 3. (for this case, it's Testing2)

Error - The site collection could not be restored

I encountered this error while restoring a site collection in a separate content database in another server. The complete error message is:

"The site collection could not be restored. If this problem persists, please make sure the content database are available and have sufficient free space."

Below are the steps that I've done to debug and fix the issue:

1. I checked the size of the hard disk drives and it has sufficient free space. 
2. I tried to restore it in another PC / server and I have restored it successfully. (That's why I wondered why this error happens only in that particular server)
3. I checked the design of dbo.Sites table in the newly created content db and found out that there are two missing columns, namely FullUrl and UserAccountDirectoryPath. So, I added manually these columns and after that I have successfully restored the site collection in that server.

FullUrl column

UserAccountDirectoryPath column