VOD在线视频点播系统论文(开题报告+英文文献翻译+ASP.net源代码) 第4页
based on the relational model. For instance, a relation named Accounts, record bank accounts, their balance, and type might look like:
                           Accounts
Account　Ｎｏ Balance  Type
 173921  1700.00  Checking
 251101 888.00 Savings
  … … …
Heading the columns are the three attributes: Account No, Balance, and Type. Below the attributes are the rows, or records. The first row says that account number 173921 has a balance of one thousand and seven hundreds dollars, and it is checking account .Suppose we wanted to know the balance of account 173921.We could ask this Query in SQL as follows:
SELECT balance FORM Accounts WHERE accountNo=173921
IBM’s DB2 and Oracle from the Oracle Corporation are examples of mainframe relational database management systems. Microsoft Access is a PC relational database management system. (Oracle also has a PC version).
Client-Server Architecture
Many varieties of modern software use a client-server architecture, in which requests by one process (the client) are sent to another process (the server) for execution. Database systems are no exception. In the simplest client/server architecture, the entire DBMS is a server, except for the query interfaces than interact with user and send queries or other commands across to the server. For example, relational systems generally use the SQL language for representing requests from the client to the server. The database server then sends the answer, in the form of a table or relation, back to the client. The relationship between client and server can get more complex especially when answers are extremely large. There is also a trend to put more work in the client, since the server will be a Bottleneck if there are many simultaneous database users.
New Forms of Database and Data Warehouses
Object-Oriented Database   Object-oriented databases store data as objects that can be automatically retrieved and shared. Included in the object are the processing instructions to complete each database transaction. These objects can contain various types of data, including sound, graphics, and video as well as traditional data and processing procedures. The objects can be shared and reused. These features of
Object-oriented databases promises to facilitate software development through reuse and the ability to build new multimedia application that combine multiple types of data. Benefit of Odom’s is their ability to support application for the World Wide Web, as described in the focus on Technology.
Object-oriented database are still a relatively new technology and can be much slower than relational systems for handing large quantities of data where there is a high volume of transaction processing. Hybrid object-relational data bases have been developed that combine the capability of handing large numbers of transactions found in relational Dams with the capability of handing complex relationships and new types of data found in Odom’s.
Hypermedia Database   Hypermedia database manage data differently from object oriented DBMS. but they can also contain diverse types of data. They store data as “chunks”of information, with each chunk in a separate node. Each node can contain traditional numeric or character data or whole
documents ,software programs, graphics, and even full motion video. Each node is totally independent-the nodes are not related by a predetermined organization scheme as they are in traditional database. Insured, user establishes their own links between nodes. The relationship among nodes is less structured than in a traditional DBMS. Searching for information does not have to follow a predetermined organization scheme. Users can branch directly form one node to another in any relationship they establish. For instance, a hypermedia database on automobiles might link basic product information with descriptive sales brochures, a video showing the automobile in action, and the location of authorized dealers.
Data Warehouses   Many companies have allowed their data to be stored in many separate systems that are unable to provide a console dated view of information usable company-wide. One way to address this problem is to build a data warehouse .A data warehouse is a database that consolidates data extracted from various production and operational systems into one large organization’s core transaction processing systems are reorganized and combined with other information, including historical data so that they can be used for management decision making and analysis. In most cases, the data in the data www.751com.cn be used for reporting-they can not be updated-so that the performance of the company’s underlying operational system is not affected. The focus on problem solving describes some of the benefits companies have obtained by using data warehouses.
Data Independence, Integrity, and Security
Data Independence   In database system, each program works with its own view or views of the database. If new fields are added to a database record, the DBMS preserves the existing views so that existing programs don’t have to be changed. The ability to modify the structure of the database without affecting existing programs that refer to the database is called data independence
Data Integrity    Data integrity refers to the accuracy, correctness, or validity of the data in the database. In a database system, data integrity means safeguarding the data against invalid alteration or destruction. In large on-line database system, data integrity becomes a more server problem.
Data Security   Data security refers to the protection of a database against unauthorized or illegal access or modification. This usually involves one or more level of password protection that is specified in the data dictionary. For example, a high-level password might allow a user to read from, write to, and modify the database structure, whereas a low-level password might only allow a user to read from the database.
Usually an audit trail, the recorded history of the modifications to a database, can be used to identify where and when a database was tampered with and it can also be used to restore the file to its original condition.   

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