Используйте having
предложение:
SELECT
CardCode,
CardName,
Sum(case when Year(DocDate)='2018' then DocTotal else 0 end) as Sold2018,
Sum(case when Year(DocDate)='2019' then DocTotal else 0 end) as Sold2019
FROM ORDR
Group By CardCode, CardName
having Sum(case when Year(DocDate)='2018' then DocTotal else 0 end) < Sum(case when Year(DocDate)='2019' then DocTotal else 0 end)
или используйте подзапрос и where
предложение:
select *
from (
SELECT
CardCode,
CardName,
Sum(case when Year(DocDate)='2018' then DocTotal else 0 end) as Sold2018,
Sum(case when Year(DocDate)='2019' then DocTotal else 0 end) as Sold2019
FROM ORDR
Group By CardCode, CardName
) where Sold2018 < Sold2019