Select only when first select is null

Select only when first select is null

I really dont get this, tried with coalesce() but with no result...
I have one select (very simplified to understand the problem):
select col1,
col2
from table1 t1
where table1.col1='value'
and table1.col2='value2'
and table1.col3='value3'
And i really need a result so if this select resultset is null (no result)
than the following sql select came to picture
select col1,
col2
from table1 t1
where table1.col1='another_value'
and table1.col2='another_value2'
How can i get this two in two one big select? (any syntax which is
recommended is appreciated...)