SELECT DISTINCT product_id

FROM fact_sales

WHERE (agent_id=1 AND product_code in (1))

OR (agent_id=2 AND product_code in (3))

OR (agent_id=3 AND product_code in (2))

OR (agent_id=4 AND product_code in (1))

AND & OR operators

http://www.w3schools.com/sql/sql_and_or.asp