conditional selection of 'and'

I have one query similar to the following:-

select coursenum,sectionnum,instructor from
SECTION s where (sectionnum = '001')
and coursenum LIKE 'MATH%'


but is it possible to add another 'and' condition to the above query provided a certain user parameter = 'add'

for e.g

select coursenum,sectionnum,instructor from
SECTION s where (sectionnum = '001')
and coursenum LIKE 'MATH%'
and instructor LIKE 'L%' i.e. only and only if arameter = 'add' else the above (first)query gets executed.

I do not want to use procedure for the above as I think everyting can be done using and,or,not