[SQLize]

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Enter MySQL script (temp tables/CRUD allowed, 5 sec / 5MB / 20 rows max):

create temporary table files_already_viewed (
    id integer,
    file_id integer,
    category integer);

create temporary table categories_files_join (
    cat_id integer,
    file_id integer);

insert into files_already_viewed values
    (1, 3, 5)
    ,
    (2, 2, 1)
    ,
    (3, 5, 1)
    /**/
    ;

insert into categories_files_join values
    (1, 2),
    (1, 5),
    (5, 3),
    (1, 3);

create temporary table FilesViewed1 as select * from files_already_viewed;
create temporary table FilesViewed2 as select * from files_already_viewed;
create temporary table CategoriesFiles1 as select * from categories_files_join;
create temporary table CategoriesFiles2 as select * from categories_files_join;

select file_id from (
    select distinct cf1.cat_id, cf1.file_id, 1 as customSort
    from CategoriesFiles1 cf1
    left join FilesViewed1 fv1 on (fv1.file_id = cf1.file_id)
    where (fv1.file_id is null)
    union
    select distinct cf2.cat_id, cf2.file_id, 2 as customSort
    from CategoriesFiles2 cf2
    join FilesViewed2 fv2 on (fv2.file_id = cf2.file_id)
    ) FinalResult
where (FinalResult.cat_id = 1)
order by customSort

| (permalink: cw372C2k3m) Sample 1 | Sample 2

No results are displayed if 5 second timeout reached.

file_id
2
5
3

(3 rows returned)