Exact or less specific match ?

Hi.

i have table like this:

create table my_data (
cond_1 int8,cond_2 varchar(),cond_3 .... cond_n whatrver ,data text)

This table represents a simple tree structure with known max level (n) .
This table is filled with data, but branches have not a same depth.

Now I need to select from table

select data from my_data where cond_1=x AND cond_2='blah' And cond_3= .....
AND cond_n=whatewer

But, this tree have not a same depth in all his branches. So I need to
select Exact match, and, if the exact match is not possible (ie if there is
not line that fit WHERE condition ),

to select with WHERE cond_1=x AND cond_2='blah' And cond_3= ..... AND
cond_(n-1)=whatewer and so on until the 'data' is not empty or top of the
tree reached (ie if not match, find data from upper node of the tree).

I know, that similar effects can be reached with COALESCE,

select coalesce ((select data from my_data where cond_1=x AND cond_2='blah'
And cond_3= ..... AND cond_n=whatewer),(select data from my_data where
cond_1=x AND cond_2='blah' And cond_3= ..... AND cond_(n-1)=whatewer)
,...........,(select data from my_data where cond_1=x ))

but i think it is not ideal, because it needs to perform a N subselects,
what can eat a lot of machine time...

is there some other way to do exact or less specific match ?

Thank you. please execuse my bad english