* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 知识管理的IT实现 IBM知识管理解决方案
Survey
Document related concepts
Expense and cost recovery system (ECRS) wikipedia , lookup
Business intelligence wikipedia , lookup
Tandem Computers wikipedia , lookup
Data vault modeling wikipedia , lookup
Relational algebra wikipedia , lookup
Oracle Database wikipedia , lookup
Concurrency control wikipedia , lookup
Microsoft Access wikipedia , lookup
Entity–attribute–value model wikipedia , lookup
Versant Object Database wikipedia , lookup
Clusterpoint wikipedia , lookup
Microsoft SQL Server wikipedia , lookup
Open Database Connectivity wikipedia , lookup
Transcript
Passage Two. Relational Database and Structured Query Language Training target: In this part,our target is to train your reading speed. You should pay more attention to” word group”. Treating a group of words as a whole unit is a short cut to save your time. We have underlined the word groups in some sentences,and the rest can be practiced by yourself. If there are some new words,you may cover the note area with a piece of paper and try to guess their meanings without looking them up in the dictionary. Text 1.Relational Database A database is similar to a data file in that it is a storage place for data. Like a data file,a database does not present information directly to a user,the user runs an application that accesses data from the database and presents it to the user in an understandable format. .. Although there are different ways to organize data in a database, relational databases are one of the most effective. Relational database systems are an application of mathematical set theory to the problem of effectively organizing data. In a relational database, data is collected into tables(called relations in relational theory). . A table represents some class of objects that are important to an organization. For example,a company may have a database with a table for employees,another table for customers,and another for stores. Each table is built of columns and rows. Each column represents some attribute of the object represented by the table. For example,an Employee table would typically have columns for attributes such as first name,last name,employee ID,department, pay grade,and job title. Each row represents an instance of the object represented by the table. For example,one row in the Employee table represents the employee who has employee ID 12345. .When organizing data into tables,you can usually find many different ways to define tables. Relational database theory defines a process called normalization,which ensures that the set of tables you define will organize your data effectively. 2.Structured Query Language normalization To work with data in a database,you have to use a set of commands and statements(language)defined by the DBMS software. Several different languages can be used with relational databases,the most common is SQL.SQL is an abbreviation of structured query language, and pronounced either See Kwell or as separate letters. SQL is a standardized query language for requesting information from a database. The original version called SEQUEL(structured English query language)was designed by an IBM research center in 1974 and 1975. . SQL was first introduced as a commercial database system in1979by Oracle Corporation. The American National Standards Institute (ANSI)and the International Standards Organization(ISO)define software standards,including standards for the SQL language. SQL Server2000supports the Entry Level of SQLˉ92,the SQL standard published by ANSI and ISO in1992.The dialect of SQL supported by Microsoft SQL Server is called TransactˉSQL(TˉSQL).TˉSQL is the primary language used by Microsoft SQL Server applications. . Historically,SQL has been the favorite query language for database management systems running on minicomputers and mainframes. Increasingly,however,SQL is being supported by PC database systems because it supports distributed databases(databases that are spread out over several computer systems).This enables several users on a local area network to access the same database simultaneously. Although there are different dialects of SQL,it is nevertheless the closest thing to a standard query language that currently exists. In1986,ANSI approved a rudimentary version of SQL as the official standard,but most versions of SQL since then have included many extensions to the ANSI standard.In1991,ANSI updated the standard. The new standard is known as SAG SQL(SQL Access Group). SQL contains about60commands and is used to create,modify, query,and access data organized in tables. It can be used either as an interactive interface or as embedded commands in an application: (1)Dynamic SQL statements are interactive,and they can be changed as needed. If you normally access SQL from a commandline environment,you are using dynamic SQL,which is slower than static SQL but much more flexible. (2)Static SQL statements are coded into application programs, and as a result,they do not change. This statements are usually processed by a precompiler before being bound into the application. SQL is such a popular standard today,every major client/server application supports it. Many database implement SQL queries behind the scenes,enabling communication with database servers in systems with client/server architecture. . The End