What is the role of SC Ganguly in the match on 2008/4/18?
role refers to of Role_Id; SC Ganguly refers to Player_Name = 'SC Ganguly'; on 2008/4/18 refers to Match_Date = '2008-04-18'
SELECT T2.Role_Id FROM Player AS T1 INNER JOIN Player_Match AS T2 ON T1.Player_Id = T2.Player_Id INNER JOIN Rolee AS T3 ON T2.Role_Id = T3.Role_Id INNER JOIN Match AS T4 ON T2.Match_Id = T4.Match_Id WHERE T1.Player_Name = 'SC Ganguly' AND T4.Match_Date = '2008-04-18'