PLS-00428 Why SELECT must be adorned with REF CURSOR to work?

Question

hello
*"PLS-00428: an INTO clause is expected in this SELECT statement"* - This problem has been resolved. I just want to understand WHY SELECT statements need to be paired with REF CURSOR.
          To reproduce this,
               DECLARE
                    Id1 numeric(19,0);
                    Id2 numeric(19,0);
                    CreateTimestamp date;
               BEGIN
               -- ATTN: You'd either need to select into variable or open cursor!
               select * from SystemUser left join Person on Person.Id = SystemUser.PersonId WHERE Person.PrimaryEmail in ('[email protected]','[email protected]');
               END;
          Solution?
               * In install script:
                    CREATE OR REPLACE PACKAGE types
                    AS
                         TYPE cursorType IS REF CURSOR;
                    END;
               * Then in your query:
                    DECLARE
                         Id1 numeric(19,0);
                         Id2 numeric(19,0);
                         Result_cursor types.cursorType;
                    BEGIN
                    OPEN Result_cursor FOR select * from SystemUser left join Person on Person.Id = SystemUser.PersonId WHERE Person.PrimaryEmail in ('[email protected]','[email protected]');
                    END;
I Googled for reasonable explaination - closest is: http://www.tek-tips.com/viewthread.cfm?qid=1338078&page=34
That in oracle block or procedures are expected to do something and a simple SELECT is not!! (Very counter intuitive). What needs to be done is therefore to put the select output into a ref-cursor to fool oracle so that it thinks the procedure/block is actually doing something. Is this explanation right? Sounds more like an assertion than actually explaining...
Any suggestion please? Thanks!

Answer 1

Opening a cursor (ref cursor or otherwise) is not the same as executing a select statement.
A select statement returns data, so if you are using it inside PL/SQL code it has to return that data into something i.e. a local variable or structure. You can't just select without a place for the data to go.
Opening a cursor issues the query against the database and provides a pointer (the ref cursor), but at that point, no data has been retrieved. The pointer can then be used or passed to some other procedure or code etc. so that it may then fetch the data into a variable or structure as required.
It's not counter-intuitive at all. It's very intuitive.