如何确定关联记录的平均数量

Question

我有两个表：

• 评论（id、created_at、completed_at、类型）
• 评论技能（id、review_id、注释）

我有以下查询告诉我评论中有 1 个或多个注释的百分比。

SELECT  TRUNC(DATE_PART('day', CURRENT_DATE - r.created_at )/7)  AS weeks_ago,
        date(min(r.created_at)) AS "Date Start",
        date(max(r.created_at)) AS "Date End",
        count(*) as  "reviews in Cohort",
                AVG(has_note::int) as "Reviews w 1 or more Notes Ratio"
FROM (SELECT r.id, r.created_at,
             ( MAX(rs.note) IS NOT NULL ) as has_note
      FROM reviews r JOIN
           reviews_skills rs
           ON r.id = rs.review_id

            WHERE r.completed_at IS NOT NULL
                    AND r.created_at > '2019-01-01'
                    AND r.type = 'long_form'

            GROUP BY r.id
     ) f
GROUP BY weeks_ago
ORDER BY weeks_ago DESC;

我要添加到此查询中的是：每次评论的平均笔记数量是多少？

Answer 1

如果我理解正确，您只需要子查询中的计数，然后是平均值：

SELECT  TRUNC(DATE_PART('day', CURRENT_DATE - r.created_at )/7)  AS weeks_ago,
        date(min(r.created_at)) AS "Date Start",
        date(max(r.created_at)) AS "Date End",
        count(*) as  "reviews in Cohort",
        AVG(has_note::int) as "Reviews w 1 or more Notes Ratio",
          AVG(num_notes) as avg_notes
FROM (SELECT r.id, r.created_at,
             COUNT(rs.note) as num_notes,
             ( MAX(rs.note) IS NOT NULL ) as has_note
      FROM reviews r JOIN
           reviews_skills rs
           ON r.id = rs.review_id
      WHERE r.completed_at IS NOT NULL AND
            r.created_at > '2019-01-01' AND
            r.type = 'long_form'
      GROUP BY r.id
     ) f
GROUP BY weeks_ago
ORDER BY weeks_ago DESC;