Yes, yes, yes.

0. Make the right kind of table. create table user_apps (user_id integer, app_id integer, primary key (user_id, app_id)); -- this does the validation for you, and (should) index the table on both user_id and (user_id,app_id).

1. Make the right kind of insert statement! insert into user_apps select 5, 6 union all select 5, 7 union all select 5, 8; -- etc

2. How do you do recommendations? That sounds like it'd be tough to do well, especially within a web request.