You can use Left Outer Join in Ax 2012 following way. There is no Right Outer join in Ax 2012. Default outer joint is left outer join so there is not left keyword is used in query.
while select AccountNum from LedgerTable order by AccountNum
outer join * from LedgerTrans where LedgerTrans.AccountNum ==
LedgerTable.AccountNum
{
info(strfmt("%1-%2", LedgerTable.AccountNum, LedgerTrans.AmountMst));
}