如果没有另外指定，结果中的列名将由union的第一个查询确定。因此，此查询应该满足您的要求：

select b.app_user_id,b.username,a.message_content,a.message_to,a.message_date 
from app_messages a 
left join app_users b 
on a.message_from = b.app_user_id
where a.message_to=1 and b.app_user_id= 4
UNION 
select b1.app_user_id,b1.username,a1.message_content,a1.message_to,a1.message_date 
from app_messages a1 
left join app_users b1 
on a1.message_from = b1.app_user_id 
where a1.message_to=4 and b.app_user_id= 1 
order by message_date;

您可以在下面尝试-您需要在外部查询中使用ORDERBY子句

select * from
(
select b.app_user_id,b.username,a.message_content,a.message_to,a.message_date 
from app_messages a 
left join app_users b 
on a.message_from = b.app_user_id
where a.message_to=1 and b.app_user_id= 4
UNION 
select b1.app_user_id,b1.username,a1.message_content,a1.message_to,a1.message_date 
from app_messages a1 
left join app_users b1 
on a1.message_from = b1.app_user_id 
where a1.message_to=4 and b1.app_user_id= 1 
)A order by message_date