--1
DROP TABLE IF EXISTS CountryMedians;
CREATE TEMPORARY TABLE CountryMedians AS
SELECT country, PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY daily_vaccinations) AS median_vaccinations
FROM vaccinations
WHERE daily_vaccinations IS NOT NULL
GROUP BY country;

--2
UPDATE vaccinations
SET daily_vaccinations = (SELECT median_vaccinations FROM CountryMedians WHERE CountryMedians.country = vaccinations.country)
WHERE daily_vaccinations IS NULL
AND EXISTS (SELECT 1 FROM CountryMedians WHERE CountryMedians.country = vaccinations.country);

--3
UPDATE vaccinations
SET daily_vaccinations = 0
WHERE daily_vaccinations IS NULL;