1、毕业设计（论文）外文翻译 1 Database and Data Warehouses What is a Database? What is a database? A database is a collection of related data items. It is generally stored on secondary storage devices that allow rapid direct access to individual data items. Redundancy is minimized； where possible, only a single co
2、py if a data item exists. The database may be used by many different application systems at once, eliminating the need for separate systems to maintain the data for each application .When a user program inquires if a particular item is in the database, a database management system (DBMS) does the ac
3、tual searching. The user does not need to be familiar with the format in which the data is stored or the actual physical location of the data. The DBMS cerates the database, keeps it up-to-date, and provides reday access to authorized user. Database management systems also provide extensive security
4、 measures to prevent unauthorized access. They make it convenient for expressing relationship between related data items and facilitate the design of user application systems. They provide backup and recovery capabilities to prevent against loss or destruction of vital information. They ensure datab
5、ase integrity, that is, what is supposed to be in the database is there and what is not supposed to be .isnt. a person called the database administrator determines who may access the database, modify it, add new relationships ,and the like. This is a very important lifeline. The real question in dat
6、abase management systems is how to organize information to provide rapid answers to the kinds of questions users are likely to ask. The same data organized differently can yield drama-tidally different access speeds. There common ways are used to structure a database to indicate the relationships am
7、ong the data items； these are the relational database, the hierarchical data-base, and the network database. Hierarchical, Network, and Relational Databases (three principal logical database models) The Hierarchical Model In a hierarchical database, data records are arranged in a strict parent child
8、 relationship. Each parent record may have many children, but each child record has exactly one parent. Figure 3-1 shows a simple hierarchical database, indicating the relationship between a customer and the orders it has placed with a company. Searching a hierarchical database is rapid and convenie
9、nt as long as it is searched from the top down. IBMs IMS (Information 毕业设计（论文）外文翻译 2 Management System) is the most widely used hierarchical DBMS. Hierarchical DBMS are best suited for problems that require a limited number of structure answers that can be specified in advance. Once data relationshi
10、ps have been specified, they cannot easily be changed without a major programming effort. Thus, the hierarchical model cannot respond flexibly to changing requests for information. The Network Model The network database model is best at representing many-to-many relationship among data. In other wor
11、ds, a ”child” can have more than one “parent”, as Figure3-2 illustrates. Computer Associates IDMS is a network DBMS for computer mainframes. Network DBMS are more flexible than hierarchical DBMS, but access paths must still be specified in advance. There are practical limitations to the number of li
12、nks, or relationships ,that can be established among records. If they are too numerous, the software will not work efficiently. Neither network no hierarchical database management models can easily create new relationships among data elements or new patterns of access without major programming effor
13、ts. The Relational Model In the early 1970s the relational database approach was developed to provide a much more user-friendly organization. Instead of using complex structures of pointers the relational database stores information simply as tables called relations. These tables are sometimes calle
14、d flat because the rows of the table really are very much the same as the records of a file. Each row in a relation is called a record. Each column corresponds to a particular field within the record (fields are also called domains).The fields are headed by attributes, which describe the entries in
15、the column. A relational database consists of one or more relations. A relational DBMS performs three primary operations on relations to form new relations: (1) Two relations may be joined (combined). (2) A relational may be projected (some of the columns are extracted form the relations and used to
16、 form the columns of the new relation). (3) Records may be selected according to various user specified criterias(Structured Query Language) is the most important query language based on the relational model. For instance, a relation named Accounts, record bank accounts, their balance, and type migh
17、t look like: 毕业设计（论文）外文翻译 3 Accounts Account 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 thous
18、and 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 IBMs DB2 and Oracle from the Oracle Corporation are examples of mainframe relational data
19、base 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 serve
20、r) 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 languag
21、e 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
22、 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
