We don't suggest you do that. It's much, much, much more straightforward to get at the contact data in CSV or vCard format via the REST interface.
However, if you really want to go straight to the database, the contact data is in the METADATA column of the MAIL_ITEM table. Contacts are rows for which TYPE = 6. To decode this data, convert to a string (the column data is in UTF-8) and then use the
BEncoding decoder of your choice.