create table my_tab(acc varchar2(10), period varchar2(10), revenue number)

insert into my_tab values('100', 'jan-18', 100)

insert into my_tab values('100', 'feb-18', 200)

insert into my_tab values('100', 'mar-18', 300)

insert into my_tab values('101', 'jan-18', 150)

insert into my_tab values('101', 'feb-18', 250)

insert into my_tab values('101', 'mar-18', 350)

commit

select * from my_tab 
pivot(sum(revenue) rev 
for period in ('jan-18' Jan, 'feb-18' Feb , 'mar-18' Mar) 
)