计算总数和部分

Question

我在 PostgreSQL 引擎驱动的数据库中有一个表 answers，其中有一个与问题相关联的数字。我需要数一数有多少个数字，按问题分组有多少个低于 6。

我想要的是这样的：

SELECT question, count(*) AS Qanswers, count_below_6(*) AS Qanswers_below_6
FROM answers
GROUP BY question;

      question         | Qanswers | Qanswers_below_6
-----------------------+----------+------------------
How do you feel?       |  1234    |      53
Was clear the webinar? |  8444    |      20
How much that hurt?    |  3666    |     142

目前我正在做

SELECT question, count(*) AS Qanswers
FROM answers
GROUP BY question;

然后

SELECT question, count(*) AS Qanswers
FROM answers
WHERE value < 6
GROUP BY question;

之后我手动合并这两个结果。
我可以用一个句子来给出我需要的结果吗？

Answer 1

SELECT question
      ,count(*) AS answers
      ,count(value < 6 OR NULL) AS under_6
FROM   answers
GROUP  BY question;

诀窍在于count(expression) 只计算非空值（而count(*) 计算所有 行）。以下是我们为什么在正确的地方得到 NULL 的详细解释：
Compute percents from SUM() in the same SELECT sql query

有许多等效的技术可以实现这一点。与基准的详细比较：
For absolute performance, is SUM faster or COUNT?