I self-joined the posts table to pair parent posts with their replies, then filtered on the timestamp window.
Clarify the schema and define 'reply' as a post that references another post via a parent_post_id. Then, for each post, count replies within 7 days of its creation, filter to posts with at least 2 such replies, and finally count distinct authors of those posts.
Pro tip: Mention that you would confirm whether replies are stored in the same posts table (self-join) or a separate replies table, and discuss how to handle time zones and edge cases like exactly 7 days.
Confirm the structure of the posts and users tables, and define what constitutes a 'reply' (e.g., a post with a parent_post_id). Also clarify the time window: 'within 7 days' likely means reply_created_at <= post_created_at + INTERVAL '7 days'.
Write a subquery that joins the posts table to itself (or to a replies table) on the reply's parent_post_id = original post's post_id, filters replies created within 7 days of the original post, groups by original post, and counts distinct replies (or reply authors) having count >= 2.
From the subquery result, select the distinct user_id (author) of the original posts and count them. Ensure you use COUNT(DISTINCT user_id) to avoid double-counting users who authored multiple qualifying posts.
Consider edge cases: replies by the original author, deleted posts, time zone differences, and whether 'within 7 days' includes the 7th day. Also, discuss performance implications and potential indexing strategies.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
The denominator is the part that almost got me.
Start by clarifying the definitions of key terms (e.g., 'reply', 'US-based user', 'distinct') and the time frame. Then, outline a step-by-step SQL or data manipulation plan: identify users with at least one reply, count distinct US repliers per user, and compute the percentage of users with ≥2 distinct US repliers. Finally, discuss potential edge cases and validation.
Pro tip: Mention that you would confirm whether 'US-based' is determined by user profile location or IP address, as this can significantly affect the analysis. Also, consider if replies from the same user but different accounts count as distinct—clarify this with the interviewer.
Ask clarifying questions to define 'reply', 'US-based user', 'distinct', and the time period. Confirm whether to include all posts or only certain types, and how to handle deleted users or posts.
Filter the dataset to include only users who have received at least one reply to any of their posts. This forms the denominator for the percentage calculation.
For each user in the filtered set, count the number of distinct US-based users who replied to their posts. Ensure that multiple replies from the same US user are counted only once.
Calculate the percentage of users (from step 2) who have at least 2 distinct US repliers. This is done by dividing the count of such users by the total number of users with at least one reply, then multiplying by 100.
Check for data quality issues, such as missing location data or duplicate replies. Discuss how to handle edge cases like users with no location info or replies from non-US users.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.