Я бы определил период даты в WHERE-предложении вместо перечисления их в SELECT:
SELECT toStartOfDay(datetime) day, match(hashtag, '[Cc]orona.*|COVID.*') ? 'COVID19' : hashtag as hashtag, count() tweets_count
FROM (
/* test data */
SELECT toDateTime(data.1) datetime, data.2 hashtag
FROM (
SELECT arrayJoin([
('2020-03-01 10:10:10', 'coronavirus'),
('2020-03-01 12:12:12', 'COVID'),
('2020-03-05 10:10:10', 'StPatricksDay'),
('2020-03-15 01:01:01', 'Coronavirus')]) data)
)
WHERE datetime >= '2020-03-01 00:00:00' AND datetime < '2020-04-01 00:00:00'
GROUP BY day, hashtag;
/* result
┌─────────────────day─┬─hashtag───────┬─tweets_count─┐
│ 2020-03-01 00:00:00 │ COVID19 │ 2 │
│ 2020-03-15 00:00:00 │ COVID19 │ 1 │
│ 2020-03-05 00:00:00 │ StPatricksDay │ 1 │
└─────────────────────┴───────────────┴──────────────┘
*/
SELECT (match(hashtag, '[Cc]orona.*|COVID.*') ? 'COVID19' : hashtag) as hashtag, sum(day1) day1, sum(day2) day2, sum(day3) day3, sum(day4) day4, sum(day5) day5, sum(day6) day6, sum(day7) day7
FROM (
/* test data */
SELECT data.1 AS hashtag, data.2 AS day1, data.3 AS day2, data.4 AS day3, data.5 AS day4, data.6 AS day5, data.7 AS day6, data.8 AS day7
FROM
(
SELECT arrayJoin([
('coronavirus', 67299, 60633, 53780, 55375, 59866, 27150, 47824),
('COVID', 62502, 50998, 50365, 51554, 50062, 23140, 40908),
('BhulaDungaFirstLook', 35524, 34, 0, 0, 0, 0, 0),
('Coronavirus', 14076, 15297, 12321, 16496, 16263, 7028, 9975),
('CoronavirusOutbreak', 13020, 9410, 2597, 1044, 1853, 950, 2436),
('BhulaDungaWithSid', 12190, 47, 0, 0, 0, 0, 0),
('StPatricksDay', 10426, 374, 244, 233, 282, 79, 213),
('QuarantineLife', 10110, 1477, 56, 0, 1, 0, 0),
('COVID2019', 9892, 2085, 1417, 2009, 2929, 1568, 4918)]) AS data
))
GROUP BY hashtag;
/* result
┌─hashtag─────────────┬───day1─┬───day2─┬───day3─┬───day4─┬───day5─┬──day6─┬───day7─┐
│ COVID19 │ 166789 │ 138423 │ 120480 │ 126478 │ 130973 │ 59836 │ 106061 │
│ StPatricksDay │ 10426 │ 374 │ 244 │ 233 │ 282 │ 79 │ 213 │
│ QuarantineLife │ 10110 │ 1477 │ 56 │ 0 │ 1 │ 0 │ 0 │
│ BhulaDungaFirstLook │ 35524 │ 34 │ 0 │ 0 │ 0 │ 0 │ 0 │
│ BhulaDungaWithSid │ 12190 │ 47 │ 0 │ 0 │ 0 │ 0 │ 0 │
└─────────────────────┴────────┴────────┴────────┴────────┴────────┴───────┴────────┘
*/