Efficient way to load referenced data in one query - sqlite

Question

My application uses a database to save its data. I have table Objects that looks like
localID | title | content
1 Test "1,embed","3,embed","5,append"
and another table Contents that looks like
localID | content
1 Alpha
2 Beta
3 Gamma
4 Delta
5 Epsilon
The main applications runs in the main thread, the whole database stuff in a second thread. So if my application loads, I want to pass each record (QSqlRecord) to the main thread where it gets further processed (loaded into real objects). I pass that record via signals. But my data is split up into 2 tables. I want to return a record containing both, perhaps similar to a join:
localID | title | content
1 Test "Alpha,embed","Gamma,embed","Epsilon,append"
So this way, I would have all the needed information at once after only one thread return value. Without combining, I would have to call the database for each single referenced content.
I expect the database to contain less than 100.000 records, yet some content may be big (files saved as blob, e.g. a book of size of 300 mb or so).
I have two questions:
(How) Can I join the tables this way inside a query (efficiently)?
Am I too concerned about threading and should make it single threaded?
That way I would not need to bother with multiple read requests.
As a sidenode, this is my first post on Database Admins, I was not too sure about this site or Stackoverflow being the right place to ask this.

Answer 1

For any actual problem, use the way recommended by #Vérace in the comments,
i.e. a "linking" table. That is the way.
However, if you are either forced to keep the database structure
or for fun
or for learning (which is indicated by the migration header),
learning dirty tricks however, instead of good design...
have a look at this:
select
localID, title,
(
with recursive cnt(x) as
( select ','||a.content
union all
select replace(x, '"'||b.localID||',', '_"'||b.content||',')
from cnt, toy2 as b
)
select replace('_"'||replace(x, ',_"', ',"'), '_","', '"') from cnt
where not x like '%,"%' LIMIT 1
) as 'content' from toy as a;
using a recursive method to flexibly
(no assumptions on number of entries in AlphaBeta table, or number of their uses)
replace the numbers by greek
applying a naming scheme with "_" to create an end condition
prepend a "_" to content, to make it be processed
and cooperate with end condition
cleanup the end-condition "_"s for desired output
cleanup special case at start of output line
select the result of recursive together with other desired outputs
Note the assumption that your table does not naturally contain '__"' or '_"'. If that happens choose more "weird" strings there. If you have all kinds of strings in your table, then you look at a very meek example of what Verace describes as "a desaster to happen". Actually this non-trivial solution is in itself probably a desaster which happened.
Output (.headers on and .mode column):
localid title content
---------- ---------- --------------------------------------------
1 Test "Alpha,embed","Gamma,embed","Epsilon,append"
2 mal "Beta,append","Delta,embed"
Here is my mcve (.dump), with an additional line "mal" for testing purposes:
BEGIN TRANSACTION;
CREATE TABLE toy (localid int, title varchar(20), content varchar(100));
INSERT INTO toy VALUES(1,'Test','"1,embed","3,embed","5,append"');
INSERT INTO toy VALUES(2,'mal','"2,append","4,embed"');
CREATE TABLE toy2 (localID int, content varchar(10));
INSERT INTO toy2 VALUES(1,'Alpha');
INSERT INTO toy2 VALUES(2,'Beta');
INSERT INTO toy2 VALUES(3,'Gamma');
INSERT INTO toy2 VALUES(4,'Delta');
INSERT INTO toy2 VALUES(5,'Epsilon');
COMMIT;
SQLite 3.18.0 2017-03-28 18:48:43