SELECT t1.art,
sum(
case when t2.date > CONVERT(DATE,'01.01.2021') then t2.sales else NULL end
) as Условие1,
sum(
case when t2.date < CONVERT(DATE,'02.02.1989') then t2.sales else NULL end
) as Условие2
FROM table1 as t1
LEFT JOIN table2 as t2
ON t1.art = t2.art
GROUP BY t1.art