查询中的 PostgreSQL 分区不起作用

Question

所以我有一个真正的休息日什么的，但对于我的生活，我无法弄清楚这一点。我想分tier1由total获得总数的百分比。我是这样想的：

(tier1 / total) as per

但没有运气。我将这些 CAST 作为 int 并且仍然没有。我会得到一个错误并且查询不会执行，或者我会得到一个值 0。

这是我正在尝试使用的内容：

SELECT count(student_id) as total
,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 1) as tier1
,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 2) as tier2
,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 3) as tier3
FROM national_assessments.aimsweb
WHERE general_outcome_measure = 'PSF' AND fall_september_tier IS NOT NULL

我也有这个想法，但没有运气：

SELECT tier1,total, tier1/total as test
FROM (
    SELECT count(student_id) as total
    ,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 1) as tier1
    ,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 2) as tier2
    ,(SELECT count(fall_september_tier) FROM national_assessments.aimsweb WHERE general_outcome_measure = 'PSF' AND fall_september_tier = 3) as tier3
    FROM national_assessments.aimsweb
    WHERE general_outcome_measure = 'PSF' AND fall_september_tier IS NOT NULL
) as test

Answer 1

整数除法截断小数位。您的表达式返回一个介于 0 和 1 之间的比率，该比率始终被截断为 0。

要获得“百分比”，首先乘以 100。
要获得小数位数，请转换为numeric（在除法之前） - 或乘以100.0。数字文字中小数位的存在会numeric自动强制结果。

通常，您会四舍五入为两个小数位或其他数字。round()为此使用（仅适用于numeric）。

向您获得的查询添加一些其他基本优化：

SELECT total
      ,tier1, round((tier1 * 100.0) / total, 2) AS tier1_pct
      ,tier2, round((tier2 * 100.0) / total, 2) AS tier2_pct
      ,tier3, round((tier3 * 100.0) / total, 2) AS tier3_pct
FROM  (
   SELECT count(*) AS total
         ,count(fall_september_tier = 1 OR NULL) AS tier1
         ,count(fall_september_tier = 2 OR NULL) AS tier2
         ,count(fall_september_tier = 3 OR NULL) AS tier3
   FROM   national_assessments.aimsweb
   WHERE  general_outcome_measure = 'PSF'
   AND    fall_september_tier IS NOT NULL
   ) sub;

计数技术说明：

• 对于绝对性能，SUM 更快还是 COUNT？