SQL SERVER 2008 – Database diagram support objects cannot be installed because this database does not have a valid owner

Full Error : Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.

Solution : You can fix this error if you have full permission of database -

ALTER AUTHORIZATION ON DATABASE::YourDatabaseName TO sa

GO 

Thanks