CASE 与…没有区别 WHERE ，它将根据数据中的每一行进行检查（除非此列上有索引）。所以我能想到的最好的办法就是

UPDATE t
SET Sample = ABS(Sample)
WHERE Sample = (SELECT MAX(Sample) FROM t WHERE Sample < 0)

编辑：只要读到你实际上需要一个“最大负片数”，如果你知道这个值是多少，你可以手动使用 CASE ```
SELECT CASE WHEN Sample = -469 THEN ABS(Sample) ELSE Sample END AS Sample
FROM t

自动执行相同操作（即找到负片的最大值并进行更改）：

WITH MaxNeg AS (SELECT MAX(Sample) as v FROM t WHERE Sample < 0)
SELECT CASE WHEN Sample = MaxNeg.v THEN ABS(Sample) ELSE Sample END AS Sample
FROM t

如果要更改行中的值。您可以使用以下查询。

update 
    t
set
    sample = abs(sample) 
where 
     sample = (select max(t.sample) from t where t.sample < 0)

但是，如果您只需要检索负片上的最大值的绝对值，那么就可以使用下面的查询。我真的认为那条 where 没有可怕的东西，你可以用它。

select abs(max(t.sample)) from t where t.sample < 0

更新
如果您只能使用select语句，那么下面的决定可以帮助您。

with 
t as 
(
    select 1281 as sample from dual union all
    select -469 as sample from dual union all
    select -469 as sample from dual union all
    select -509 as sample from dual union all
    select -1359 as sample from dual union all
    select -1359 as sample from dual union all
    select -2759 as sample from dual union all
    select -2829 as sample from dual
)
select 
    case 
        when t.sample = (select max(t.sample) from t where t.sample < 0) then
            abs(t.sample)
        else
            t.sample
    end sample            
from 
    t

这个查询的结果是：

sample
-------
1281
469
469
-509
-1359
-1359
-2759
-2829

你可以试试下面的sql，它过滤掉所有的负数。然后按负数降序排列，只选择第一行。

update tab
set tab.val = ABS(tab.val)
WHERE
    tab.val < 0
ORDER BY tab.val DESC;