Something like this maybe?

select letter from

(select 'a' letter

union

select 'b' letter

union

select 'c' letter

union

select 'd' letter

union

select 'e' letter)source

order by (case letter when 'd' then 1when 'c' then 2 when 'b' then 3 when 'a' then 4 when 'e' then 5 end)