每当您开始考虑要本地化的GROUP BY时，都应该开始考虑窗口函数。
我认为您正在追求这样的事情：
select unit_id, time, diag
from (
select unit_id, time, diag,
rank() over (partition by unit_id order by time desc) as rank
from diagnostics.unit_diag_history
) as dt
where rank = 1
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