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You can find the physical data model on the following thread: http://www.magentocommerce.com/boards/viewthread/500/
It’s not bad to get the overall picture, but won’t help much for a detailed understanding of every table.
Not really database related, but there also Moshe’s reference document about Magento architecture that may help you understand the page request flow:http://www.magentocommerce.com/wiki/doc/magento-architecture