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【正文】 n time ? Analogous to the value of a variable ? Physical Data Independence – the ability to modify the physical schema without changing the logical schema ? Applications depend on the logical schema ? In general, the interfaces between the various levels and ponents should be well defined so that changes in some parts do not seriously influence others. 169。 city : integer。Silberschatz, Korth and Sudarshan Database System Concepts Purpose of Database System ? In the early days, database applications were built on top of file systems ? Drawbacks of using file systems to store data: ? Data redundancy and inconsistency ? Multiple file formats, duplication of information in different files ? Difficulty in accessing data ? Need to write a new program to carry out each new task ? Data isolation — multiple files and formats ? Integrity problems ? Integrity constraints (. account balance 0) bee part of program code ? Hard to add new constraints or change existing ones 169。169。Silberschatz, Korth and Sudarshan Database System Concepts Purpose of Database Systems (Cont.) ? Drawbacks of using file systems (cont.) ? Atomicity of updates ? Failures may leave database in an inconsistent state with partial updates carried out ? . transfer of funds from one account to another should either plete or not happen at all ? Concurrent access by multiple users ? Concurrent accessed needed for performance ? Uncontrolled concurrent accesses can lead to inconsistencies – . two people reading a balance and updating it at the same time ? Security problems ? Database systems offer solutions to all the above problems 169。 end。Silberschatz, Korth and Sudarshan Database System Concepts Data Models ? A collection of tools for describing ? data ? data relationships ? data semantics ? data constraints ? EntityRelationship model ? Relational model ? Other models: ? objecto
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