Use an UNION query, something like
SELECT TDate, Sum(L) As Longs, Sum(S) As Shorts, Sum(A) As Amount FROM
(
SELECT TDate, Count(*) As L, 0 As S, Sum(Amount) As A FROM tblTrade WHERE Type = "L" GROUP BY TDate
UNION ALL
SELECT TDate, 0 As L, Count(*) As S, Sum(Amount) As A FROM tblTrade WHERE...