I have a query created in MS access that have the following code (expressions) whats the equivalent of these expressions in sqlplus

Select DateSerial(Mid([SPMFINV]![INV_DATE],1,4),Mid([SPMFINV]![INV_DATE],5,2),Mid([SPMFINV]![INV_DATE],7,2)) AS InvDate

and

select payment, IIf([PAY_DATE]>0,DateSerial(Mid([PAY_DATE],1,4),Mid([PAY_DATE],5,2),Mid([PAY_DATE],7,2)),DateSerial(Mid([SPMFINV]![INV_DATE],1,4),Mid([SPMFINV]![INV_DATE],5,2),Mid([SPMFINV]![INV_DATE],7,2))+IIf(Val([PAYMNT_DAY])=0,30,Val([PAYMNT_DAY]))) AS DueDate,


What acn i use in place of the MID and IIF in SQLPLUS