WITH MedianValues AS (
    SELECT
        country,
        PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY daily_vaccinations) AS median_vaccinations
    FROM
        country_vaccination_stats
    WHERE
        daily_vaccinations IS NOT NULL
    GROUP BY
        country
),
ZeroCountries AS (
    SELECT DISTINCT
        country,
        0 AS median_vaccinations
    FROM
        country_vaccination_stats
    WHERE
        country NOT IN (SELECT country FROM MedianValues)
)
UPDATE country_vaccination_stats
SET daily_vaccinations = COALESCE(mv.median_vaccinations, zc.median_vaccinations)
FROM country_vaccination_stats cvs
LEFT JOIN MedianValues mv ON cvs.country = mv.country
LEFT JOIN ZeroCountries zc ON cvs.country = zc.country
WHERE daily_vaccinations IS NULL;