My first instinct was a regular join and I almost wrote it that way before catching myself.
Start by clarifying the table schemas and the definition of a reply (e.g., whether replies can be nested or deleted). Then use a LEFT JOIN from posts to replies, group by post, and count replies, ensuring zero-reply posts are included. Finally, apply the specified ordering with a tie-breaker on post_id.
Pro tip: Mention that using COUNT(replies.id) instead of COUNT(*) avoids counting NULLs from the LEFT JOIN, and explicitly state the ordering logic to show attention to detail.
Confirm the structure of the posts and replies tables, including primary/foreign keys, and whether replies can be deleted or nested. Ask if the reply count should include all replies or only top-level ones.
Use a LEFT JOIN from posts to replies to ensure all posts are included, even those without replies. Alternatively, consider a correlated subquery or a pre-aggregated CTE for performance.
Group by post_id (and any other post columns needed) and count the replies using COUNT(replies.id) to handle NULLs correctly. Alias the count as reply_count.
Order the results by reply_count descending, then by post_id ascending to meet the tie-breaking requirement.
Mentally test with posts having zero replies, multiple replies, and ties in reply counts. Ensure the query returns the expected results and discuss potential performance considerations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Two ways to do this and I went with NOT EXISTS instead of a LEFT JOIN + WHERE NULL check.
Identify the posts table and the replies table (or a self-referencing parent_post_id column) in the given schema, then use a LEFT JOIN or NOT EXISTS subquery to find posts with no matching replies. Filter out posts that have at least one reply and order the remaining post IDs ascending.
Pro tip: Clarify whether 'no replies' includes deleted or hidden replies; if so, add a filter to exclude them. Also, consider performance: NOT EXISTS is often more efficient than LEFT JOIN with NULL check on large datasets.
Identify the posts table and the replies table (or self-referencing column) and the key that links replies to posts.
Decide between LEFT JOIN with IS NULL, NOT EXISTS, or NOT IN based on performance and null-handling requirements.
Construct the SQL query to select post IDs from posts where no matching reply exists, ensuring correct join condition.
Add ORDER BY post_id ASC and consider any additional filters (e.g., exclude deleted posts).
Check for edge cases (e.g., posts with replies that are deleted) and ensure the query uses indexes efficiently.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.