如何从非规范化表计算非虚增计数 [英] How to calculate a non-inflated count from a denormalized table

问题描述

假设我有一个非规范化表，该表包含一个ID和一个我需要计数的值。像这样的东西：

Suppose I have a denormalized table that includes an ID and a value that I need to count. Something like this:

 Tree_ID | ...other columns... |  Count_If_True
------------------------------------------------
       1 | ...other values...  |           True
       1 | ...other values...  |           True
       2 | ...other values...  |           True
       2 | ...other values...  |           True
       3 | ...other values...  |           True

在这种情况下，从表组中选择Tree_ID，count（Count_If_True）由Tree_ID 显示：

 Tree_ID |  count(Count_If_True)
---------------------------------
       1 |                     2
       2 |                     2
       3 |                     1

但是如果我通过来自 Apples的联接进一步规范化我的表表（其中每棵树都有多个苹果），它看起来像这样：

But If I denormalize my table further with a join from an Apples table (where every tree has multiple apples), it would look something like this:

Apple_ID | Tree_ID | ...other columns... |  Count_If_True
------------------------------------------------
       1 |       1 | ...other values...  |           True
       2 |       1 | ...other values...  |           True
       3 |       1 | ...other values...  |           True
       4 |       1 | ...other values...  |           True
       5 |       1 | ...other values...  |           True
       6 |       1 | ...other values...  |           True
       7 |       2 | ...other values...  |           True
       8 |       2 | ...other values...  |           True
       9 |       2 | ...other values...  |           True
      10 |       2 | ...other values...  |           True
      11 |       2 | ...other values...  |           True
      12 |       2 | ...other values...  |           True
      13 |       2 | ...other values...  |           True
      14 |       2 | ...other values...  |           True
      15 |       3 | ...other values...  |           True
      16 |       3 | ...other values...  |           True
      17 |       3 | ...other values...  |           True
      18 |       3 | ...other values...  |           True
      19 |       3 | ...other values...  |           True

这会将我们的计数膨胀为：

This would inflate our count to:

 Tree_ID |  count(Count_If_True)
---------------------------------
       1 |                     6
       2 |                     8
       3 |                     5

是否有一种简单的方法（例如，没有CTE）编写单个查询以获取返回引入 Apple_ID 之前的原始计数结果？

Is there a simple way (without a CTE, for example) to write a single query to get back the original count result before Apple_IDs were introduced?

推荐答案

您在第一个表中需要一个唯一的行标识符-也许在其他列中。它可以是一列或多列。然后，您可以使用 count（distinct）：

You need a distinct row identifier in the first table -- perhaps that is among the other columns. It can be one or more columns. Then you can use count(distinct):

select tree_id,
       count(distinct <unique row column>) filter (where count_if_true)
from t
group by tree_id;

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