我只想做：

SELECT (case when volume = 0 then '0' when volume < 21 then '1-20' when volume>=21 then '21+' end) as Volume,
       sum(ab) as c1,
       (case when volume = 0
             then sum(sum(ab)) over () - sum(sum(xy))
             else sum(xy)
       end) as c2
FROM  table t
GROUP BY (case when volume=0 then '0' when volume <21 then '1-20' when volume>=21 then '21+' end);

当您可以简单地使用窗口函数时，子查询和cte是不必要的。

WITH x
AS
(
  SELECT 
    case when volume=0 then '0' when volume <21 then '1-20' when volume>=21 then '21+' as Volume,
    sum(ab) as c1,
    sum(xy) as c2
  FROM 
    table t
  GROUP BY
    CASE when volume=0 then '0' when volume <21 then '1-20' when volume>=21 then '21+'
)
SELECT x.Volume, x.c1, (CASE WHEN x.Volume = 0 
                             THEN (SELECT SUM(x2.c1 - x2.c2) 
                                   FROM x AS x2) 
                             ELSE x.c2 END)
FROM x

您可以使用下面的查询来获得所需的输出，并使用可以实现的cte表表达式。
cte为（从表t中依次选择大小写，当体积=0时，选择“0”，当体积<21时，选择“1-20”，当体积>=21时，选择“21+”，求和（ab）为c1，求和（xy）为c2，求和（ab）-求和（xy）为c，当体积=0时，选择“0”，当体积<21时，选择“1-20”，当体积>=21时，选择“1-20”，然后选择“21+”结束）
选择volume，c1，case when volume='0'，然后（从cte中选择sum（c3）），否则从cte中选择c2 end c2