The Sugar team has recently released ER (entity relationship) diagrams for the Sugar 5.0 database. We will make ER diagrams available with each release moving forward.
The package includes a PDF document and a DDL (data definition language) file that creates the schema with foreign key relationships. You can use the DDL to reverse engineer the diagrams with any diagram tool such as MySQL WorkBench.
You will find the Sugar 5.0 Community Edition ER diagrams here in the Developer Zone. The ER diagrams for the Sugar Professional and Sugar Enterprise Editions are available for subscribers in the download manager at http://support.sugarcrm.com.