如何在 Postgres ROLLUP 中区分真正的 NULL 和正式的 NULL？

问题描述

我试图确定每个给定行所属的 ROLLUP 报告的“级别”。当初始数据不包含 NULL 时，则可以只计算每一行中的空值（每个给定级别/分组层的空单元格数量相同），尽管在我看来它也不是一个漂亮的解决方案。>

如果初始数据包含 NULL，则此解决方法不再适用：在示例查询结果表中，您会看到 <null> 来自真实的 Null，而 (null) 是分组单元格的标准 NULL 占位符.

我希望找到类似于“分层查询中的虚拟列（如 LEVEL、PATH 等）”的内容。

Postgresql v12 中是否有一种自然的方法来确定使用 ROLLUP（通常使用 GROUPING SETS）构建的报告中的行级别？

ROLLUP 查询结果示例：

select a,b,sum(d.c)
from(
   select null as a,2 as b,1 as c
   union all
   select null as a,3 as b,1 as c
   union all
   select 'a1' as a,4 as b,5 as b,1 as c
   union all
   select 'a2' as a,6 as b,1 as c
 ) d GROUP by ROLLUP (d.a,d.b)


|   a    | b      | sum  |
--------------------------
| (null) | (null) |  5   |
|   a1   |   5    |  1   |
| <null> |   2    |  1   |
|   a1   |   4    |  1   |
|   a2   |   6    |  1   |
| <null> |   3    |  1   |
|   a2   | (null) |  1   |
| <null> | (null) |  2   |
|   a1   | (null) |  2   |
-------------------------- 

解决方法

使用分组，有点

select case when grouping(a) = 1 then 'Total' else cast(a as varchar(20)) end a,case when grouping(b) = 1 then 'Total' else cast(b as varchar(20)) end b,sum(d.c)
from(
   select null as a,2 as b,1 as c
   union all
   select null as a,3 as b,1 as c
   union all
   select 'a1' as a,4 as b,5 as b,1 as c
   union all
   select 'a2' as a,6 as b,1 as c
 ) d GROUP by ROLLUP (d.a,d.b)
 order by grouping(a),a,grouping(b),b