【正文】 se, delete or change the structure of the database. When a procedure or failure of the puter database affect or damage the database may also be reconstruction. This step usually is the longest in the database development process step, as it continued to databases and related applications throughout the life cycle, the development of each database can be seen as a brief database development process and data modeling concepts arise, logical and physical database design and database to achieve dealing with the changes. 3 database development of the threetier architecture model In this article on the front of the database development process mentioned in the interpretation of a system development project on the establishment of the several different, but related database view or model: ● conceptual model (in the analysis stage of the establishment). ● external model or user view (in the analysis phase and the establishment of logical design phase). ● physical model or internal model (in the physical design phase of the establishment). Figure 7 describes the database view that the relationship between the three, it is important to remember that they are the same anizations database view or model. In other words, each anization has a database of the physical model, a concept model and one or more users view. Therefore, the threetier architecture model using the same data set observe the different ways definition database. Concept models on the full database structure, has nothing to do with the technical specifications. Conceptual model definition do not involve the entire database data stored in the puter how the secondary memory. Usually, the conceptual model by entities links (ER) map or object modeling symbols such a graphical format to describe, we have this type of concept model called the data model. In addition, the conceptual model specification as a metadata stored in the database or data dictionary. Physical models including conceptual model of how data stored in puter memory in the two specifications. Analysts and the database design is as important to the physical database (physical mode) definition, it provides information on the distribution and management of data storage and access of the physical memory space of two full database technology specifications. Database development and database technology database is among the three models divided into basis. Database development projects may have a role to only deal with these three views of a related work. For example, a beginner may be designed for one or more procedures external model, and an experienced developer will design the physical model or conceptual model. Database design issues at different levels are quite different. 三、 threetier structure of the database positioning system Obviously, all the good things in the database are, and the three! When designing a database, you have to choose where to store data. This option in the physical database design stage. Database is divided into individual databases, the Working Group database, departmental databases, corporate databases and the Inter database. Individuals often by the enduser database design and development of their own, just by database experts to give training and advice to help, it only contains individual endusers interested in the data. Sometimes, personal database from the database or enterprise Working Group extracted from the database, such circumstances database prepared by some experts from the regular routine to create local database. Sector Working Group database and the database is often the enduser, business experts and the central database system experts development. The collaborative work of these officers is necessary because in the design of the database to be shared by a large number of issues weigh: processing speed, ease of use, data definition differences and other similar problems. Due to corporate databases and the Inter database broad impact, largescale, it is normally concentrated in the database development team has received professional training to develop a database of experts. 1. Customers layer A desktop or notebook also known as that layer, which specialized management user interface and system localization data in this layer can be implemented on the Web scripting tasks. 2. Server / Web server HTTP protocol handling, scripting tasks, the implementation of puting and provide data access, the layer known as processing services layer. 3. Enterprise Server (Miniputer or mainframe) layer The implementation of plex puting and interanizational management from multiple data sources of data integration, also known as data services layer. In an anization, hierarchical database and information system architecture for distributed puting and the client / server architecture of the concept of correlation. Client / server architecture based on a LAN environment, including servers (referred to as database server or database engine) database software implementation from the client workstation database orders, each customer applications focus on their user interface functions. In fact, the whole concept of the database (as well as the application of these databases to handle routine) as a distributed database or the separate but related physical database distribution in the local PC workstation, server intermediate (working group or sector) and one center server (departments or enterprises ). Simply said that the use of client / server architecture for: ● it can handle multiple processors on the same application at the same time, improve application response time and data processing speed. ● It can use each puter platform of the best data processing (such as PC Mini Advanced user interface with the mainframe and puting speed). ● can mix various