SP2007 to 2010 database upgrade received SPUpgradeException

I have simply exported WSS_Content database on SP2007 farm to SP2010 farm with SQL Server Management Studio and renamed it as WSS_Content_Imported. I also created a new Web Application with default empty content database WSS_Content on SP2010 farm.

Opened PowerShell and run as Administrator.

Test-SPContentDatabase -Name WSS_Content_Imported -WebApplication http://sp

Lots of errors. However, all UpgradeBlockings are False. Then I tried to upgrade with

Dismount-SPContentDatabase -Identity WSS_Content

Success.

Mount-SPContentDatabase -Name WSS_Content_Imported -WebApplication http://sp

Failed. It returns:

Mount-SPContentDatabase : Exception of type 'Microsoft.SharePoint.Upgrade.SPUpg
radeException' was thrown.
At line:1 char:24
+ Mount-SPContentDatabase <<<<  -Name WSS_Content_Imported -WebApplication http
://sp
    + CategoryInfo          : InvalidData: (Microsoft.Share...ContentDatabase:
   SPCmdletMountContentDatabase) [Mount-SPContentDatabase], SPUpgradeExceptio
  n
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletMountCon
   tentDatabase