如何区分 Postgres ROLLUP 中的真实 NULL 和正式 NULL？答案

【问题标题】：How to distinguish between real NULL and formal NULL in Postgres ROLLUP?如何区分 Postgres ROLLUP 中的真实 NULL 和正式 NULL？
【发布时间】：2021-09-04 13:00:27
【问题描述】：

我正在尝试确定每个给定行所属的 ROLLUP 报告的“级别”。当初始数据不包含空值时，可以只计算每一行中的空值（对于每个给定的分组级别/层，空单元格的数量是相同的），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，则此解决方法不再有效：在示例查询结果表中，您会看到 <null> 来自真正的 null，而 (null) 是分组单元格的标准 NULL 占位符。

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

PostgreSQL v12 中是否有一种自然的方式来确定使用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, 1 as c
   union all
   select 'a1' as a, 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   |
--------------------------

【问题讨论】：

标签： sql postgresql postgresql-12 grouping-sets

【解决方案1】：

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, 1 as c
   union all
   select 'a1' as a, 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

【讨论】：