Oracle SQL联接/合并2个表并按ID分组/排序以创建类似Facebook的时间轴

Question

For a piece of coursework I have to create a facebook-like application.

Currently I am working on the 'timeline' page and struggling with the complexity of the SQL query that is required.

My desired outcome is to display posts and responses in a single table and group or sort so that the responses are below the corresponding post

eg post_id        text
     1          First post
     1          Response 1 to post 1
     1          Response 2 to post 1
     2          Second post
     2          Response 1 to post 2
     2          Response 2 to post 2

Database schema is as follows:

create or replace type post_obj as object (
   post_id number,
   post_text varchar2(500),
   user_posted_by ref user_obj,
   post_date timestamp,
   map member function get_postid return number) not final;

create table post_obj_tbl of post_obj;


create or replace type response_obj as object (
   response_id number,
   response_text varchar2(500),
   post_details ref post_obj,
   user_posted_by ref user_obj,
   response_date timestamp,
   map member function get_responseid return number) not final;

create table response_obj_tbl of response_obj;

So far I have managed to get the two tables to join so that I get a single table displaying posts and responses but it is automatically grouping by the user id. Here is the query:

SELECT p_tbl.user_posted_by.username, p_tbl.post_text, p_tbl.post_date
FROM post_obj_tbl p_tbl

WHERE 

p_tbl.user_posted_by.user_id = (
   SELECT u_tbl.user_id
   FROM user_obj_tbl u_tbl 
   WHERE u_tbl.username = (select V('APP_USER') from DUAL) )

OR

p_tbl.user_posted_by.user_id in (
   SELECT f_tbl.friend_id 
   FROM friends_obj_tbl f_tbl
   WHERE f_tbl.user_id = (
      SELECT u_tbl.user_id
      FROM user_obj_tbl u_tbl 
      WHERE u_tbl.username = (select V('APP_USER') from DUAL) )
   )

UNION
SELECT r_tbl.user_posted_by.username, r_tbl.response_text, r_tbl.response_date
FROM response_obj_tbl r_tbl, post_obj_tbl p_tbl
WHERE r_tbl.post_details.post_id = p_tbl.post_id;

I really hope this makes sense...it's messing with my head! Any help much appreciated :)

Answer 1

First of all, why do you create tables of objects? Usual way is to create tables like this:

create table post_obj  (
   post_id number,
   post_text varchar2(500),
   user_posted_by number,
   post_date timestamp);

It is much easier to work with such table, especially in APEX.
About query. I'll take one part:

SELECT p_tbl.user_posted_by.username, p_tbl.post_text, p_tbl.post_date
FROM post_obj_tbl p_tbl
WHERE 
p_tbl.user_posted_by.user_id = (
   SELECT u_tbl.user_id
   FROM user_obj_tbl u_tbl 
   WHERE u_tbl.username = (select V('APP_USER') from DUAL) )

It can be transformed to:

SELECT p_tbl.user_posted_by.username, p_tbl.post_text, p_tbl.post_date
FROM   post_obj_tbl p_tbl,
       user_obj_tbl u_tbl
WHERE  p_tbl.user_posted_by = u_tbl.user_id
   AND u_tbl.username = V('APP_USER')