Left join with nested selects and aggregate functions - sql

Question

Problem
I have one table of generated dates (s) which I want to join with another table (d) which is a list of dates where a specific occurrence has happened.
table s
Wednesday 23rd August 2017
Thursday 24th August 2017
Friday 25th August 2017
Saturday 26th August 2017
table d
day_created -------------------------------- count
Thursday 24th August 2017 ---------------- 45
Saturday 26th August 2017 ---------------- 32
I want to show rows where the occurrence does not take place, which I cannot do if I just have table d.
I want something that looks like:
day_created -------------------------------- count
Wednesday 23rd August --------------------- 0
Thursday 24th August 2017 ---------------- 45
Friday 25th August 2017 ------------------ 0
Saturday 26th August 2017 ---------------- 32
I've tried joining with a left join as follows:
SELECT day_created, COUNT(d.day_created) as total_per_day
FROM
(SELECT date_trunc('day', task_1.created_at) as day_created
FROM task_1
)
d
LEFT JOIN (
SELECT (generate_series('2017-05-01', current_date, '1 day'::INTERVAL)) as standard_date
)
s
ON d.day_created=s.standard_date
GROUP BY d.day_created
ORDER BY day_created DESC;
I don't get an error however the join isn't working (i.e. it doesn't return dates where the count is null). What it returns is the dates from table d and the count, but not the dates in between where there are 0 occurrences.
I've been going round in circles and have understood that I need to make table s (I think!) the left table, but I'm getting confused as a newbie with the syntax.
This is all in PostgreSQL 9.5.8.

Answer 1

Basically, you had the LEFT JOIN backwards. This should work, with some other simplifications and performance optimizations:
SELECT s.standard_date, COUNT(d.day_created) AS total_per_day
FROM generate_series('2017-05-01', current_date, interval '1 day') s(standard_date)
LEFT JOIN task_1 d ON d.day_created >= s.standard_date
AND d.day_created < s.standard_date + interval '1 day'
GROUP BY 1
ORDER BY 1;
This counts rows in d, like you commented. Does not sum values.
Be aware that generate_series() still returns timestamp with time zone, even if you pass date values to it. You may want to cast to date or format with to_char() for display in the outer SELECT. (But rather group and order by the original timestamp value, not the formatted string.)
There may be corner cases depending on the current time zone setting depending on the actual undisclosed table definition.
Related:
How to avoid a subquery in FILTER clause?

Answer 2

I have one table of generated dates (s)
In real databases, we don't store a generated series. We just generate them when needed.
which I want to join with another table (d) which is a list of dates where a specific occurrence has happened. [...] I want to show rows where the occurrence does not take place, which I cannot do if I just have table d.
Nah, you can do it.
CREATE TABLE d(day_created, count) AS VALUES
('24 August 2017'::date, 45),
('26 August 2017'::date, 32);
SELECT day_created, coalesce(count,0)
FROM (
SELECT d::date
FROM generate_series(
'2017-08-01'::timestamp without time zone,
'2017-09-01'::timestamp without time zone,
'1 day'
) AS gs(d)
) AS gs(day_created)
LEFT OUTER JOIN d USING(day_created)
ORDER BY day_created;
day_created | coalesce
-------------+----------
2017-08-01 | 0
2017-08-02 | 0
2017-08-03 | 0
2017-08-04 | 0
2017-08-05 | 0
2017-08-06 | 0
2017-08-07 | 0
2017-08-08 | 0
2017-08-09 | 0
2017-08-10 | 0
2017-08-11 | 0
2017-08-12 | 0
2017-08-13 | 0
2017-08-14 | 0
2017-08-15 | 0
2017-08-16 | 0
2017-08-17 | 0
2017-08-18 | 0
2017-08-19 | 0
2017-08-20 | 0
2017-08-21 | 0
2017-08-22 | 0
2017-08-23 | 0
2017-08-24 | 45
2017-08-25 | 0
2017-08-26 | 32
2017-08-27 | 0
2017-08-28 | 0
2017-08-29 | 0
2017-08-30 | 0
2017-08-31 | 0
2017-09-01 | 0
(32 rows)