Discussion of the database schemas happens either in the forum of the relevant module, or in the Databases forum.
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Moodle 1.8
http://moodle.org/file.php/5/moddata/forum/28/376698/moodle_1.8_data_structure.rar
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Moodle 1.7
http://moodle.org/file.php/11/moddata/forum/110/268325/db-edudist-Moodle-1.7_.rar
[edit]
Moodle 1.5.3
http://moodle.org/file.php/11/moddata/forum/110/267644/Db-Edudist-Moodle1.5.3-xml.rar