update

scripts/activity_description.sql

 create table activity_description_export as SELECT id, comment_external FROM `activity`
 WHERE comment_external is not null;
+
+update civicrm_event e inner join civicrm_value_act a on a.entity_id = e.id inner join activity_description_export ae on ae.id = a.act_id set e.description = ae.comment_external;
+
+drop table activity_description_export;