PostgreSQL query to count/group by day and display days with no data

Question

I need to create a PostgreSQL query that returns

• a day
• the number of objects found for that day

It\'s important that every sing

Answer 1

Extending Gordon Linoff's helpful answer, I would suggest a couple of improvements such as:

• Use ::date instead of date_trunc('day', ...)
• Join on a date type rather than a character type (it's cleaner).
• Use specific date ranges so they're easier to change later. In this case I select a year before the most recent entry in the table - something that couldn't have been done easily with the other query.
• Compute the totals for an arbitrary subquery (using a CTE). You just have to cast the column of interest to the date type and call it date_column.
• Include a column for cumulative total. (Why not?)

Here's my query:

WITH dates_table AS (
    SELECT created::date AS date_column FROM sharer_emailshare WHERE showroom_id=5
)
SELECT series_table.date, COUNT(dates_table.date_column), SUM(COUNT(dates_table.date_column)) OVER (ORDER BY series_table.date) FROM (
    SELECT (last_date - b.offs) AS date
        FROM (
            SELECT GENERATE_SERIES(0, last_date - first_date, 1) AS offs, last_date from (
                 SELECT MAX(date_column) AS last_date, (MAX(date_column) - '1 year'::interval)::date AS first_date FROM dates_table
            ) AS a
        ) AS b
) AS series_table
LEFT OUTER JOIN dates_table
    ON (series_table.date = dates_table.date_column)
GROUP BY series_table.date
ORDER BY series_table.date

I tested the query, and it produces the same results, plus the column for cumulative total.

Answer 2

Based on Gordon Linoff's answer I realized another problem was that I had a WHERE clause that I didn't mention in the original question.

Instead of a naked WHERE, I made a subquery:

SELECT d.date, count(se.id) FROM (
    select to_char(date_trunc('day', (current_date - offs)), 'YYYY-MM-DD')
    AS date 
    FROM generate_series(0, 365, 1) 
    AS offs
    ) d 
LEFT OUTER JOIN (
    SELECT * FROM sharer_emailshare 
    WHERE showroom_id=5
) se
ON (d.date=to_char(date_trunc('day', se.created), 'YYYY-MM-DD')) 
GROUP BY d.date;

Answer 3

You just need a left outer join instead of an inner join:

SELECT d.date, count(se.id)
FROM (SELECT to_char(date_trunc('day', (current_date - offs)), 'YYYY-MM-DD') AS date 
      FROM generate_series(0, 365, 1) AS offs
     ) d LEFT OUTER JOIN
     sharer_emailshare se 
     ON d.date = to_char(date_trunc('day', se.created), 'YYYY-MM-DD'))  
GROUP BY d.date;

Answer 4

I like Jason Swett SQL however ran into issue where the count on some dates should be a zero rather than a one. Running the statment select count(*) from public.post_call_info where timestamp::date = '2020-11-23' count = zero, but below equals a one.

Also the + give me a forward schedule so changed to a minus provide 9 days data prior to current date.

SELECT sequential_dates.date,
COUNT(*) AS call_count
FROM (SELECT CURRENT_DATE - sequential_dates.date AS date
        FROM generate_series(0, 9) AS sequential_dates(date)) sequential_dates
LEFT JOIN public.post_call_info ON public.post_call_info.timestamp::date = 
    sequential_dates.date
GROUP BY sequential_dates.date
    order by date desc

Answer 5

I'll try to provide an answer that includes some explanation. I'll start with the smallest building block and work up.

If you run a query like this:

SELECT series.number FROM generate_series(0, 9) AS series(number)

You get output like this:

 number 
--------
      0
      1
      2
      3
      4
      5
      6
      7
      8
      9
(10 rows)

This can be turned into dates like this:

SELECT CURRENT_DATE + sequential_dates.date AS date
  FROM generate_series(0, 9) AS sequential_dates(date)

Which will give output like this:

    date    
------------
 2019-09-29
 2019-09-30
 2019-10-01
 2019-10-02
 2019-10-03
 2019-10-04
 2019-10-05
 2019-10-06
 2019-10-07
 2019-10-08
(10 rows)

Then you can do a query like this (for example), joining the original query as a subquery against whatever table you're ultimately interested in:

   SELECT sequential_dates.date,
          COUNT(calendar_items.*) AS calendar_item_count
     FROM (SELECT CURRENT_DATE + sequential_dates.date AS date
             FROM generate_series(0, 9) AS sequential_dates(date)) sequential_dates
LEFT JOIN calendar_items ON calendar_items.starts_at::date = sequential_dates.date
 GROUP BY sequential_dates.date

Which will give output like this:

    date    | calendar_item_count 
------------+---------------------
 2019-09-29 |                   1
 2019-09-30 |                   8
 2019-10-01 |                  15
 2019-10-02 |                  11
 2019-10-03 |                   1
 2019-10-04 |                  12
 2019-10-05 |                   0
 2019-10-06 |                   0
 2019-10-07 |                  27
 2019-10-08 |                  24