【71-75】A database system is a ollction of itererelated data and a set of programs that allow users to access and modify these data. A major purpose of a database system is to provide users with an（71）view of the data. That is, the system hides certain details of how the data are stored and maintained. For the system to be usable, it must retrieve data（72）。 The need for efficiency has led designers to use complex data（73）to represent data in the database. Since many database-system users are not computer trained, developers hide the complexity from users through several levels of abstraction, to simplify users' interactions with the system. Physical level is the lowest level of abstraction that describes（74）the data are actually stored. Logical level is the next-higher level of abstraction that describes（75）data are stored in the database, and what relationships exist among those data. View level is the highest level of abstraction that describes only part of the entire database.
