计算机专业英语Unit-6--Database.ppt

,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,单击此处编辑母版标题样式,Computer English,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,单击此处编辑母版标题样式,Computer English,Unit 5 Database,Section A Database,Background Knowledge,背景知识,数据库（,Database,）是按照数据结构来组织、存储和管理数据的仓库，它产生于距今六十多年前，随着信息技术和市场的发展，特别是二十世纪九十年代以后，数据管理不再仅仅是存储和管理数据，而转变成用户所需要的各种数据管理的方式。,一般认为数据管理技术的发展经历了四个阶段：人工管理阶段、文件系统阶段、数据库阶段和高级数据库技术阶段。,New Words and Useful Expressions,retrieve/rtrv/,重新得到,收回,;,检索,撷取,distinct/dstkt/,清楚的,;,截然不同的,;,明显的,duplicate/djuplkt/,复制,;,影印,拷贝,;,复写,;,重复,identifier/adentfa(r)/,识别符,;,检验人，标识符,conceivably/knsi:vbl/,令人信服地,composite/kmpzt/,混合的，组合的,restriction/rstrkn/,限制,;,约束,criterion/kratrn/,(,复数,)criteria,。标准,;,规范,;,准据,clause/klz/,子句,;,条款,New Words and Useful Expressions,wildcard/waldk:d/,通配符,elaborate/lbrt/,精细的,复杂的,详尽的,nested/nestd/,嵌套的,mileage/mald/,英里数,;,运费,metadata/metdet/,元数据,cascade/ksked/,小瀑布,;,波浪；（计）级联,cursor/krs/,游标，指针,iterative/tretv/,反复的,;,絮叨的,rollback/rlbk/,回滚,undo/ndu/,复原,取消一个动作,New Words and Useful Expressions,up-to-date,更新,integrity constraint,完整性约束,keep track of,记录,iterative processing,迭代处理,data concurrency,数据并发,stored procedure,存储过程,Relational Database&DBMS,A relational database is one that presents information in tables with rows and columns.,A Database Management System(DBMS)handles the way data is stored,maintained,and retrieved.,Integrity Rules,First,the rows in a relational table should all be distinct,A second integrity rule of the traditional relational model is that column values must not be repeating groups or arrays.,A third aspect of data integrity involves the concept of a null value.Two null values are not considered equal.,SQL,Structured Query Language,SQL is a language designed to be used with relational databases,SELECT A1,，,A2,，,，,An,（指出目标表的列名或列表达式序列）,FROM R1,，,R2,，,，,Rm,（指出基本表或视图序列）,WHERE F,（,F,为条件表达式）,GROUP BY,列名序列 （结果表分组）,ORDER BY,列名排序方向,SQL-WHERE,A SELECT statement,also called a query,is used to get information from a table.,The WHERE clause in a SELECT statement provides the criteria for selecting values.,no WHERE clause:no restrictions,The WHERE clause in a SELECT statement provides the criteria for selecting values.,WHERE clauses can get rather elaborate with multiple conditions and nested conditions in some DBMSs.,Using the table name and a dot(.)before the column name indicates which table contains the column.,Foreign Key,Foreign Key:,There must be one column that appears in both tables in order to relate them to each other.This column,which must be the primary key in one table,is called the foreign key in the other table.,A foreign key must either be null or equal to an existing primary key value of the table to which it refers.This is different from a primary key,which may not be null.,Referential Integrity,Referential integrity is a property of data which,when satisfied,requires every value of one attribute(column)of a relation(table)to exist as a value of another attribute in a different(or the same)relation(table).,SQL Command Category,SQL commands are divided into categories,the two main ones being Data Manipulation Language(DML)commands and Data Definition Language(DDL)commands.,DML commands deal with data,either retrieving it or modifying it to keep it up-to-date.DDL commands create or change tables and other database objects such as views and indexes.,DDL Commands,CREATE TABLE,DROP TABLE,ALTER TABLE,CREATE TEMPORARY TABLE tbl_name,(create_definition,.),table_options select_statement,Result Set&Cursor,The rows that satisfy the conditions of a query are called the result set,A user can access the data in a result set one row at a time,and a cursor provides the means to do that.,Cursor:created automatically,move both forward and backward,Transactions&Lock,Transactions:maintain data in a consistent state(data consistency)while allowing more than one user to access a database at the same time.,Transaction result:commit or rollback,depending on whether there are any problems with data consistency or data concurrency,Lock:prohibits two transactions from manipulating the same data at the same time,Stored Procedures,a group of SQL statements that can be called by name,executable code,store it in the database,DBMS-specific programming language,Java programming language,etc,Metadata,information about the database itself.,Most DBMSs have a set of system tables,which list tables in the database,column names in each table,primary keys,foreign keys,stored procedures,and so forth,In general,developers writing tools and drivers are the ones most likely to be concerned with metadata.,Reading MaterialIntroduction to Typical Databases,The Web provides access to a variety of data that can be multimedia-rich.Readily available information retrieval techniques such as inverted indices,which allow efficient keyword-based access to text,largely enabled access to the exponentially growing Web.,Todays DBMS technology faces yet another challenge as researchers attempt to make sense of the immense amount of heterogeneous,fast-evolving data available on the Web.,