Select all values from last date that is shared between rows - sql

Question

I’ve got a Postgresql table with counts for countries over time. Not every country has a count for each date, and some have NULL values. I’d like to get the counts for each country up to the last date every country has data for, excluding NULL values.
I made a DB Fiddle with example data.
Example:
country date count id
Germany 2020-05-25 10 1
Germany 2020-05-26 11 2
Germany 2020-05-27 12 3
Germany 2020-05-28 13 4
Italy 2020-05-25 20 5
Italy 2020-05-26 21 6
Italy 2020-05-27 22 7
Italy 2020-05-28 23 8
France 2020-05-25 30 9
France 2020-05-26 31 10
France 2020-05-27 NULL 11
I’d like to get back the following:
country date count id
Germany 2020-05-25 10 1
Germany 2020-05-26 11 2
Italy 2020-05-25 20 5
Italy 2020-05-26 21 6
France 2020-05-25 30 9
France 2020-05-26 31 10
I’ve searched, but I’m relatively new to SQL and don’t seem to know what keywords to search for.

Answer 1

You can use window functions to count the number of rows with dates and then compare to the number of countries:
SELECT c.*
FROM (SELECT c.*, COUNT(count) over (partition by date) as num_countries_on_date
FROM countries c
) c
WHERE num_countries_on_date = (SELECT COUNT(DISTINCT c2.country) FROM countries c2);
Here is a db<>fiddle.
If you wanted to generate data for a range of dates -- sort of the opposite problem -- you could use a CROSS JOIN to generate the rows, a LEFT JOIN to bring in the data, and COALESCE() to turn NULL to 0:
SELECT c.country, d.date, coalesce(co.count, 0) as count
FROM (SELECT DISTINCT country FROM countries) c CROSS JOIN
generate_series('2020-05-26'::date, '2020-05-27'::date, interval '1 day') d(date) LEFT JOIN
countries co
ON co.country = c.country AND co.date = d.date;