When using the PowerShell command Mount-SPContentDatabase or adding content database through the Central Admin you will receive this error message:
The SELECT permission was denied on the object 'sysobjects', database 'mssqlsystemresource', schema 'sys'.
if the SharePoint Admin account has the deny permissions checked. The easy fix was to open SQL Server Management Studio and modify the roles of the admin account on the content database.
Make sure that db_denydatareader and db_denydatawriter are unchecked on the user.