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
出现上述问题的原因：是因为你把另外一个服务器上的数据库导出后，对于当前数据库没有兼容好，解决如下：
运行一下sql query
EXEC sp_dbcmptlevel 'dbname', '90';
go
ALTER AUTHORIZATION ON DATABASE::[dbname] TO [username suggest 'SA']
go
use dbname
go
EXECUTE AS USER = N'dbo' REVERT
go