DBMS（Database-Management System）: a collection of interrelated data and set of programs to access those data.
Part 1: Database-System Applications
1. Enterprise Information
2. Banking and Finance
3. Universities
4. Airlines
5. Telecommunication
Part 2: Purpose of Database Systems
Keeping organizational information in a file-processing system has a number of major disadvantages:
1. Data redundancy and inconsistency
2. Difficulty in accessing data.
3. Data isolation
4. Integrity Problems
5. Atomicity Problems
Part 3: View of Data
A database system is a collection of interrelated 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 abstract view of the data. That is, the system hides certain details of how the data are stored and maintained.
1. Data Abstration
Physical level. The lowest level of abstraction describes how the data are actually stored. The physical level describes complex low-level data structures in detail.
Logical level. The next-higher level of abstraction describes what data are stored in the database, and what relationships exist among those data. The logical level thus describes the entire database in terms of a small number of relatively simple structures. Although implementation of the simple structures at the logical level may involve complex physical-level structures, the user of the logical level does not need to be aware of this complexity. This is referred to as physical data independence. Database administrators, who must decide what information to keep in the database, use the logical level of abstraction.
View level. The highest level of abstraction describes only part of the entire database. Even though the logical level uses simpler structures, complexity remains because of the variety of information stored in a large database. Many users of the database system do not need all this information; instead, they need to access only a part of the database. The view level of abstraction exists to simplify their interaction with the system. The system may provide many views for the same database.
2. Instances and Schemas
Databases change over time as information is inserted and deleted.
- The collection of information stored in the database at a particular moment is called an instance of the database.
- The overall design of the database is called the database schema.
Database systems have several schemas, partitioned according to the levels of abstraction.
The physical schema describes the database design at the physical level, while the logical schema describes the database design at the logical level. A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.