Можно попробовать следующее:
SELECT top 10 Merchant,
isnull(SUM(CASE WHEN Month(ta.dt) = '01' THEN Sales END),0) AS "Jan Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '02' THEN Sales END),0) AS "Feb Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '03' THEN Sales END),0) AS "Mar Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '04' THEN Sales END),0) AS "Apr Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '05' THEN Sales END),0) AS "May Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '06' THEN Sales END),0) AS "Jun Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '07' THEN Sales END),0) AS "Jul Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '08' THEN Sales END),0) AS "Aug Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '09' THEN Sales END),0) AS "Sep Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '10' THEN Sales END),0) AS "Oct Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '11' THEN Sales END),0) AS "Nov Sales",
isnull(SUM(CASE WHEN Month(ta.dt) = '12' THEN Sales END),0) AS "Dec Sales"
FROM tbl_Merchant ORDER BY Sales DESC
Топ-10 продаж каждого месяца будут отображаться в виде различных столбцов по месяцам.
Надеюсь, это поможет вам.