February 20, 2023

ChatGPT - PostgreSQL Query

I need a postgres sql query bucketing results by hour and joining in a timeseries so that buckets with zero count show a show a zero.

Assuming you have a table called my_table with a timestamp column called created_at, and you want to bucket results by hour, you could use the following query:

SELECT  generate_series(start_hour, end_hour, '1 hour') AS hour_bucket, count(my_table.created_at) AS count FROM  (SELECT    date_trunc('hour', MIN(created_at)) AS start_hour, date_trunc('hour', MAX(created_at)) AS end_hour FROM my_table) AS time_range LEFT JOIN  my_table ON date_trunc('hour', my_table.created_at) = generate_series(start_hour, end_hour, '1 hour') GROUP BY hour_bucket ORDER BY hour_bucket;

Here's what this query does:

  1. The inner query determines the earliest and latest hour in your data by using the MIN and MAX functions, and rounds them down to the nearest hour using the date_trunc function.
  2. The generate_series function is then used to generate a list of hours from the earliest to the latest hour in your data, in one-hour increments. This is the "timeseries" that you mentioned in your question.
  3. The LEFT JOIN then joins this timeseries with your my_table table on the condition that the created_at timestamp falls within the corresponding hour bucket in the timeseries. This will include any hour buckets with zero count.
  4. Finally, the GROUP BY and COUNT functions are used to count the number of rows in my_table that fall within each hour bucket, and the results are ordered by the hour bucket.

Note that the LEFT JOIN will include null values for hour buckets with zero count, so the output will show a NULL value in the count column for those buckets. If you want to show a zero instead, you can use the COALESCE function to replace the NULL value with a zero, like this:

SELECT  generate_series(start_hour, end_hour, '1 hour') AS hour_bucket, COALESCE(count(my_table.created_at), 0) AS count FROM  (SELECT    date_trunc('hour', MIN(created_at)) AS start_hour, date_trunc('hour', MAX(created_at)) AS end_hour FROM my_table) AS time_range LEFT JOIN  my_table ON date_trunc('hour', my_table.created_at) = generate_series(start_hour, end_hour, '1 hour') GROUP BY hour_bucket ORDER BY hour_bucket;