When upgrading from Moodle 1.9.8+ to Moodle 2.0 (Build: 20101130) the upgrade process fails when upgrading the table _role_assignments.
The error message "Debug info: Column was set to data type implicit default; NULL supplied for NOT NULL column 'itemid' at row 3" is returned with the failed query

"UPDATE mdl_role_assignments
SET itemid = (SELECT MIN(mdl_enrol.id)
FROM mdl_enrol
JOIN mdl_context ON (mdl_context.contextlevel = 50 AND mdl_context.instanceid = mdl_enrol.courseid)
WHERE mdl_role_assignments.component = CONCAT('enrol_', mdl_enrol.enrol) AND mdl_context.id = mdl_role_assignments.contextid)
WHERE component <> 'enrol_manual' AND component LIKE 'enrol_%'"

and additional info...

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Stack trace:

• line 394 of /lib/dml/moodle_database.php: dml_write_exception thrown
• line 672 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
• line 4419 of /lib/db/upgrade.php: call to mysqli_native_moodle_database->execute()
• line 1309 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 252 of /admin/index.php: call to upgrade_core()

I am able to manually run the above query on the MySql database which runs without an error. If I then continue the install process after this stage (2010061900.23) then the upgrade completes with no further errors.

This information is also in the post at http://moodle.org/mod/forum/discuss.php?d=163300