数据库课件重点ch01.pdfVIP

Database Management System (DBMS) DBMS provides an environment that is both convenient and efficient to use. It contains A collection of interrelated data, i.e. database which contains information about a particular enterprise A set of programs to access the data Database Applications: Banking: all transactions Airlines: reservations, schedules Universities: registration, grades Sales: customers, products, purchases Online retailers: order tracking, customized mendations Manufacturing: production, inventory, orders, supply chain Human resources: employee records, salaries, tax deductions Databases touch all aspects of our lives Purpose of Database Systems In the early days, database applications were bui irectly on top of file systems Drawbacks of using file systems to store data: Data redundancy and onsistency 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 (e.g. account balance 0) e “buried” in program code rather than being stated explicitly Hard to add new constraints or change existing ones Purpose of Database Systems (Cont.) Drawbacks of using file systems (cont.) Atomicity of updates Failures may leave database in an onsistent state with partial updates carried out Example: Transfer of funds from one account to another should either complete or not happen at all Concurrent access by multiple users Concurrent accessed needed for performance Uncontrolled concurrent accesses can lead to onsistencies – Example: Two people reading a balance and updating it at the same time Security problems  Hard to provide user access to some, but not all, data Database systems offer solutions to all t