Один из вариантов расчета будет:
with my_table(Issue_date, qty, unit_price, LPO) as
(
select '2018-01-10',1,42 ,'1-2018-001166' union all
select '2018-01-12',1,100,NULL union all
select '2018-09-20',1,25 ,NULL union all
select '2018-10-15',2,12 ,'1-2018-002233' union all
select '2018-10-20',1,100,'1-2018-002233'
)
select Qty,unit_price,LPO,
(case
when issue_date between '2018-01-01' and '2018-03-31'
then (((qty) *(unit_price)) * 1.05 )
when issue_date > '2018-03-31' then
( case when lpo is null then
(((qty) *(unit_price)) * 1.05 )
else
(((qty) *(unit_price)))
end )
end ) as Tot_inc_Tax
from My_Table;
Демоверсия dbfiddle