Good thing to know

so this kind of game that have demos on the psn and are basically the full game but locked and you need to buy the game to unlock a key unlock them? you know that kind of game? most of the time those guys dont work, so dont bother trying with them. for example, uncharted is that kind of game, rayman is that kind of game and LBP is that kind of game. correct me if im wrong pls