How to create database diagrams in SQL Server 2008 express?

I am using SQL Server 2008 express version but don't know how to generate database diagrams. When I click the Database Diagrams, it give me an error diagram tell me "Database d开发者_如何学Pythoniagram support objects cannot be installed because this database does no thave a valide owner. ...." I follow its instruction and cannot find anything wrong. Anybody can help me about this?

Anybody can recommend a free database design tool I can use with SQL Server to create ER diagram for our existing database?

You need to change the compatibility level and set authorisation:

Database diagram support objects cannot be installed