with prep as (
SELECT *
, LAG(confirmed_cases, 1)
OVER (PARTITION BY state_name ORDER BY date ASC) AS prev_day
FROM `bigquery-public-data.covid19_nyt.us_states`
where upper(state_name) = 'TEXAS'
and date >= '2021-03-01'
order by date asc
)

select date
, confirmed_cases - prev_day as new_cases --since the prev.day
from prep
order by date asc