使用postgres tablefunc crosstab（）计算错误答案

Question

我有一个视图（我们称其为“水果”），其中包含一列正确答案以及多项选择题考试中相关的错误答案，我想统计一下最常选择哪些错误答案（容易混淆）那些）。视图看起来像这样：

correct_answer | wrong_response
-------------------------------
apple          |   pear
apple          |   pear
apple          |   banana
banana         |   apple
banana         |   pear
banana         |   pear
banana         |   pear
pear           |   apple

我想要的是一个数据透视表，该表对正确答案的错误回答进行计数，以使列代表正确答案，而行则代表错误答案的计数。

wrong_response | apple | banana | pear
---------------------------------------
apple          | 0     | 1      | 1
banana         | 1     | 0      | 0
pear           | 2     | 3      | 0

我曾经在这里before使用过此功能，但那时候我并没有在努力计算。任何帮助将不胜感激！

编辑：对于将来的人们，这两种解决方案都可以工作！但是，条件聚合更为灵活。仅当查询中包含所有可能性时，交叉表解决方案才有效。例如，如果排除梨（或添加猕猴桃），则交叉表解决方案将返回错误。无论是否排除（或添加当前不存在的）记录，条件汇总都将返回结果。谢谢您的帮助。

Answer 1

如果您知道这些列，则可以使用条件聚合：

select wrong_response,
       count(*) filter (where correct_answer = 'apple') as apple,
       count(*) filter (where correct_answer = 'pear') as pear,
       count(*) filter (where correct_answer = 'banana') as banana
from t
group by wrong_response;

Answer 2

假设您已经完成：CREATE EXTENSION tablefunc;

然后通过crosstab（）函数实现所需的目标是：

SELECT *
FROM crosstab('SELECT wrong_response,
                      correct_answer,
                      count(*)
               FROM fruit
               GROUP BY wrong_response, correct_answer 
               ORDER BY wrong_response',

              'SELECT correct_answer
               FROM fruit
               GROUP BY correct_answer
               ORDER BY correct_answer')

AS (wrong_answer varchar(20),
    apple bigint,
    banana bigint,
    pear bigint);

以上代码将为您提供以下结果：

请注意，此处0输出为null，为了获得所需的内容，您只需要稍微修改select：

SELECT
    wrong_answer,
    coalesce(apple, 0) as apple,
    coalesce(banana, 0) as banana,
    coalesce(pear, 0) as pear
FROM crosstab('SELECT wrong_response,
                      correct_answer,
                      count(*)
               FROM fruit
               GROUP BY wrong_response, correct_answer 
               ORDER BY wrong_response',

              'SELECT correct_answer
               FROM fruit
               GROUP BY correct_answer
               ORDER BY correct_answer')

AS (wrong_answer varchar(20),
    apple bigint,
    banana bigint,
    pear bigint)

以上将使您达到所需的状态：