Introduction to Database Systems
1.1 Role of the database system
1.1.1 Data and Data Management
1) data. Symbol data recording is referred to describe things. Data is stored in the object database, the object database management system is handled. Data and interpretation of the data are inseparable.
2) data management. Data processing is the sum of a series of activities various data collection, storage, retrieval, processing, and dissemination of applications. Data management is effective data classification, organization, encoding, storage, retrieval, maintenance and application, it is the central problem of data processing.
1.1.2 emergence and development of data management technology
1) Artificial management phase. Oriented applications, a data set corresponds to only one program, no corresponding system software responsible for the management of data. When a plurality of applications some of the same design data, respectively, to be defined by the respective applications and manage the data can not be shared use, has redundant.
2) file system stage. Use file system management data is unified data management by specialized software. For a particular application, the data is centrally organized data files stored in a plurality of groups, and to develop a specific application for the file group. File management systems use technology "to access by file name, according to records Access", you can modify the file, insert and delete.
The drawbacks of the file system: poor data sharing, data redundancy and inconsistency. Data independence is poor. Data isolation. Data acquisition difficult. Integrity problems (also known as consistency constraints). Atomic issues. Concurrent access exceptions. Security issues.
3) database management system stages. Database management system is a collection of interrelated data and a set of procedures to access, manage and control the composition of the data. This data set is usually called a database.
Database management system advantages:
a. unitary structure data.
b. Sharing data is high, low redundancy, easy to expand. Data independence is high.
C. Data independence is used to describe the dependency between the data and applications, including logic and data independence of physical data independence.
Physical independence means that user applications and data stored on the disk of the database are independent.
Independence logic refers to a logical structure of the user's application and data are independent of each other.
d. Data in managed and controlled by a database management system
DBMS must provide: data security protection. Check the integrity of the data. Concurrency control. Database recovery.
1.2 Data Model
Describe a data model is a collection of data between the data link and data semantic data operation, and the consistency (integrity) concept constraint tool. Can abstract the characteristics of real-world data by the data model.
Classification 1.2.1 Data Model
Data model should meet three requirements: First, can more realistically simulate real world; the second is easy to be understood; third is easy to implement on a computer.
1) conceptual model . Also known information model, which according to the user's point of view or understanding of the real-world data and information modeling, mainly for database design. There are common conceptual model ER model, OO model.
2) logic model . Logical data abstraction layer is a middle layer, it is used to describe the logical structure of the database data. The data abstraction layer called a logical data model. It is seen by the user through the database management system in the real world, is based on the viewpoint of the computer system for data modeling, data that is realized in the form of a computer, is mainly used for the DBMS.
3) the physical model . The physical layer is the lowest level of data abstraction for the physical data storage structures and access methods described. It is not only determined by the DBMS design, but also closely related to the operating system, computer hardware.
Constituent elements 1.2.2 Data Model
1) data structure . Composite object data structure descriptor database (data) and links between objects.
2) data operation . It refers to the operation data database instances various objects in the set of allowed operations performed, including rules relating to the operation and the operation.
3) integrity constraint data . Data integrity rules are given in the model and the data link control and dependency rules has to change the state of the database and the status of the composite data model defined to ensure proper, effective and compatible data.
Data Model are: hierarchical model, network model, relational model, object-oriented model, XML model.
1.3 Data abstraction and database three modes
1. Data Abstraction
1) physical layer abstraction. The lowest level of abstraction, describing how the data is actually stored.
2) logical layer of abstraction. Description stored in the database What is the relationship between these data and what data.
3) abstract view layer. The highest level of abstraction, only to describe a part of the entire database.
2. The three modes of database
Schema is a description of all the logical data structures and database features, it is only described design type, do not involve a specific value. A specific example of a pattern of values called pattern.
Three mode refers to the outer structure of the database model of the database management system, and a mode three mode within different levels of abstraction observation angle data in the database.
1) mode. Also called logical data abstraction layer. Is a description of all data and the logical structure of the database features is a common view of all users. DBMS provides data definition language (DDL) to strictly defined pattern. Mode corresponds to the table.
2) mode. A view corresponding to the data abstraction layer, which is a logical description of the structure and characteristics of the database and use the user can see the local data.
3) mode. Also called storage mode, corresponding to the physical layer data abstraction, which is the physical structure description data and the storage of the data in the internal database representation.
Two data mapping function and independence 3. Database
1) an outer mode / image mode . It defines the correspondence between the mode and the external mode. When the mode is changed by the database administrator to change accordingly respective outer pattern image / pattern can be kept constant external mode. Ensuring the independence of data and logic program, referred to as a logical data independence.
2) mode / the mode map . When the storage structure of the database changes, the database administrator mode. The corresponding change in the mode map, can mode remains unchanged. Ensure the independence of physical data and procedures, referred to as physical data independence.
1.4 Database System
1.4.1 database system components
From the standpoint DBMS: database is a system configuration mode outer / pattern / mode of the three modes; viewed from the user: a user database system into a single structure, from the main structure, distributed architecture, client / server, browsers / application server / database server architecture.
Database contains four types of data: user data, metadata and the metadata index. A collection of user data is through the relationship between the structure of the (two-dimensional table) organization of all business data; Metadata is data about the statistical structure of the database describing the relationship between data and databases, has become a data dictionary; the index is to improve database performance accessories and data accessibility established; user application metadata forms, reports, queries, and other forms of application components.
1.4.2 database management system
1. DBMS functionality
1) Data Definition. Data definition language provided by the DBMS (Data Definition Language, DDL), the user can easily in the database of data objects defined by it.
2) data organization, storage and management. DBMS tissue classification, storage and management of various kinds of data, including data dictionary, the user data, the data access path and the like.
3) data manipulation. DBMS also provides a data manipulation language (Data Manipulation Language, DML) change user search through it deletions.
4) transaction management database and running. In establishing the database, operation and maintenance, centralized control DBMS unified management to ensure the security, integrity, multi-user concurrent operation of the data and the system after a failure data recovery.
5) the establishment and maintenance of the database. Initial data entered into the database, conversion, dump database recovery, re-organization function and performance monitoring databases, analysis functions.
2. DBMS composition
1) update mode. The logical structure of the database to be modified.
2) query.
3) update.
4) query processor. SQL operations requested by the user to query optimization.
5) Storage Manager. According to enforce policies, taking data from the database or update data.
6) transaction manager. Responsible for integrity assurance system to ensure running multiple simultaneous transactions do not conflict.