How to use `NULL` in a `where` clause - sqlite

Question

In sqlite3 I am combining entries from various tables in a new table (making use ot primary and foreign key contsraints; for details see at the bottom):
select * from files;
id file description
---------- -------------- ----------------
1 testmodel1.dat first test model
2 testmodel2.dat second test mode
select * from patches;
id patch description
---------- ---------- ------------
1 testpatch test patch
Combined table:
select * from patched_files;
id files_id patches_id
---------- ---------- ----------
1 1 NULL
2 1 1
3 2 1
Some files do get patched some not (sometimes the same file).
I would like to retrieve this view from the database (expected result):
id file patch
---------- -------------- ----------
1 testmodel1.dat
2 testmodel1.dat testpatch
3 testmodel2.dat testpatch
I tried this:
select patched_files.id as id,patched_files.files_id as file_id, files.file as file, patched_files.patches_id as patch_id, patches.patch as patch
from patched_files, files, patches
where patched_files.files_id=files.id and patched_files.patches_id=patches.id;
id files_id file patches_id patch
---------- ---------- -------------- ---------- ----------
2 1 testmodel1.dat 1 testpatch
3 2 testmodel2.dat 1 testpatch
Here, I am missing the unpatched file (rowid 1) and I understand why.
So I added an or:
select patched_files.id as id,patched_files.files_id as file_id, files.file as file, patched_files.patches_id as patch_id, patches.patch as patch
from patched_files, files, patches
where patched_files.files_id=files.id and (patched_files.patches_id=patches.id or patched_files.patches_id is NULL);
id file_id file patch_id patch
---------- ---------- -------------- ---------- ----------
1 1 testmodel1.dat NULL testpatch
2 1 testmodel1.dat 1 testpatch
3 2 testmodel2.dat 1 testpatch
I understand why this is happening (composite table from "files" and "patches").
How can I avoid the patch name being stated if the patch_id is NULL?
Do I need to insert a "dummy" in the "patches" table?
Recreating the example:
-- original files:
create table files (id integer primary key, file text, description text);
insert into files (file, description) values('testmodel1.dat', 'first test model');
insert into files (file, description) values('testmodel2.dat', 'second test model');
-- patches:
create table patches (id integer primary key, patch text, description text);
insert into patches (patch, description) values('testpatch', 'test patch');
-- patched files (combination of files and patches):
create table patched_files (id integer primary key, files_id integer references files(id) on delete restrict deferrable initially deferred, patches_id integer references patches(id) on delete restrict deferrable initially deferred);
insert into patched_files (files_id) values(1);
insert into patched_files (files_id, patches_id) values(1,1);
insert into patched_files (files_id, patches_id) values(2,1);

Answer 1

You are looking for an outer join:
select f.id, f.file, p.patch, p.description
from files f
left join patched_files pf on pf.files_id = f.id
left join patches p on p.id = pf.patches_id
Online example: http://rextester.com/ZNMAY53081