Understanding Erlang ODBC application - error-handling

Question

I'm connecting to an DB source with Erlang ODBC. My code looks like:
main() ->
Sql = "SELECT 1",
Connection = connect(),
case odbc:sql_query(Connection, Sql) of
{selected, Columns, Results} ->
io:format("Success!~n Columns: ~p~n Results: ~p~n",
[Columns, Results]),
ok;
{error, Reason} ->
{error, Reason}
end.
connect() ->
ConnectionString = "DSN=dsn_name;UID=uid;PWD=pqd",
odbc:start(),
{ok, Conn} = odbc:connect(ConnectionString, []),
Conn.
It's ok now. But how can I handle errors at least in my query? As I understand it contains in {error, Reason}, but how can I output it when something gone wrong? I'm trying to add io:format like at the first clause, but it doesn't work.
At second, unfortunately, I can't find any reference that can explain syntax well, I can't understand what does ok mean in this code (first - line 8, and second - line 16. If I'm right it just means the case that connection is ok and this variable isn't assigned? But what it means at 8 line?)

Answer 1

ok in line 8 is the return value of the case statement when the call to odbc:sql_query(Connection, Sql) returns a result that can match the expression {selected, Columns, Results}. In this case it is useless since the function io:format(...) already returns ok.
the second ok: {ok, Conn} is a very common Erlang usage: the function returns a tuple {ok,Value} in case of success and {error,Reason} in case of failure. So you can match on the success case and extract the returned value with this single line: {ok, Conn} = odbc:connect(ConnectionString, []),
In this case the function connect() doesn't handle the error case, so this code has 4 different possible behaviors:
It can fails to connect to the database: the process will crash with a badmatch error at line 16.
It connects to the database but the query fails: the main function will return the value {error,Reason}.
It connects to the database and the query returns an answer that doesn't match the tuple {selected, Columns, Results}: the process will crash with a badmatch error at line 4.
It connects to the database and the query returns an answer that matches the tuple {selected, Columns, Results}: the function will print
Success!
Columns: Column
Results: Result
and returns ok

Answer 2

So I found something. The {error, Reason} contains the connection errors, means that we specified wrong DSN name etc. Regarding to my offer to catch query error we can read this from Erlang reference:
Gaurds All API-functions are guarded and if you pass an argument of
the wrong type a runtime error will occur. All input parameters to
internal functions are trusted to be correct. It is a good programming
practise to only distrust input from truly external sources. You are
not supposed to catch these errors, it will only make the code very
messy and much more complex, which introduces more bugs and in the
worst case also covers up the actual faults. Put your effort on
testing instead, you should trust your own input.
Means that we should be careful about what we write. That's not bad.