In this paper I want to focus on some principal differences between data models of database systems and knowledge representation languages. The data manipulation languages of data models are based on the closed-world, unique-name, and domain-closure assumption. Data manipulation languages and query languages of knowledge representation formalisms differ considerably in their underlying assumptions. They are based on the open-world, unique-name, and open-domain assumption. That means, that even if the data definition language and the data manipulation language of a database management system and a knowledge base management system would coincide, the results of data manipulations would differ. I present some examples that show the usefulness of closed-world inferences in natural language processing. Thus knowledge representation languages sticking to the open-world assumption seem to be insufficient for natural language processing. [an error occurred while processing this directive]