基本上，您需要
union all：
select id, username, firstname, event_datefrom ((select id, username, firstname, event_date from tablea ) union all (select id, username, NULL as firstname, event_date from tablea ) ) torder by event_date;
基本上，您需要
union all：
select id, username, firstname, event_datefrom ((select id, username, firstname, event_date from tablea ) union all (select id, username, NULL as firstname, event_date from tablea ) ) torder by event_date;
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