Sorry, don't know any sites. But when you work in such an environment, you've got the manuals...
What I remember (although not 20 years it is some time ago) just code the normal simple ANSI SQL statement, preceeded by the adabas/natural keyword.
Looks like:
exec adabas
find
select name
from personell
into :WS-NAME
end exec.
or
exec adabas
histogram
select name
from personell
into :WS-NAME
end exec.
When you know adabas, you'll know the difference
In the output of the pre-processor you will see the adabas control blocks and various buffers being generated, resulting in a call to adabas.
Advanced relational SQL features with all kind of joins, multiple tables and so is not possible. But everything you can do with Natural you can also accomplisch with Cobol-ADA-SQL.