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1、Chapter 2 Database Environment (数据库环境),2.1 The Architecture of Database Systems 2.2 Database Languages 2.3 Data Models & Conceptual Modeling 2.4 Functions of a DBMS 2.5 Components of a DBMS 2.6 Multi-User DBMS Architectures,2.1 The Architecture of Database Systems,Early 2-level architecture, DBTG(Da
2、ta Base Task Group), 1971: a system view(schema) (1个系统视图(模式) n user views (subschemas) (n个用户视图(子模式) 3-level architecture, ANSI(American National Standards Institute) SPARC(Standards Planning and Requirements Committee), 1975: a physical view(internal schema) (1个物理视图(内模式) a conceptual view(conceptual
3、 schema) (1个概念视图(概念模式) n user views (external schemas) (n个用户视图(外模式) Schema means the description of data (“模式”意为“数据的描述”).,2.1 The Architecture of Database Systems,ANSI-SPARC three-level architecture of DBS,2.1 The Architecture of Database Systems,1. External Level 2. Conceptual Level 3. Internal Lev
4、el 4. Schemas, Mappings, and Instances 5. Data Independence,1. External Level,Consists of multiple user views (external schemas) of the DB (由多个数据库用户视图(外模式)构成: 解决需要使用哪些数据的问题). Each user view includes the part of DB relevant to the user (每个用户视图包含与该用户相关的数据库部分). Users are unaware of what is out of their
5、 views (用户感觉不到其视图以外的数据). Different views may have different representations of the same data (相同数据在不同视图中可能有不同的表示).,2.1 The Architecture of Database Systems,1. External Level 2. Conceptual Level 3. Internal Level 4. Schemas, Mappings, and Instances 5. Data Independence,2. Conceptual Level,Consists of
6、 a conceptual schema of DB (What should be stored?) (由一个数据库概念模式构成, 包含整个数据库应存储的数据. 解决应该存储哪些数据的问题). contains logical representation of the entire DB, which includes (包含整个数据库的逻辑表示, 包括): all entities, their attributes, and their relationships; constraints on data; security & integrity info. supports eac
7、h user view, in which any data must be contained in, or derived from, the conceptual schema (支持每个用户视图, 其中的任何数据都必须源自概念模式).,must not contain any storage-dependent details (概念模式中不应包含任何数据存储细节).,2.1 The Architecture of Database Systems,1. External Level 2. Conceptual Level 3. Internal Level 4. Schemas, M
8、appings, and Instances 5. Data Independence,3. Internal Level,Consists of an internal schema of DB (How to store data?) (由一个数据库内模式构成, 解决如何存储数据的问题). contains physical representation of the DB, which includes (包含数据库的物理表示，包括): storage space allocation for data and indexes (为数据和索引分配存储空间) record descript
9、ion for storage (记录的存储描述); record placement (记录的存放安排); data compression & encryption (数据压缩和加密).,3. Internal Level,Below the internal level there is a physical level, managed by OS and used by DBMS (内模式之下是一个物理层, 它由OS管理, 由DBMS使用), in which functions of DBMS and OS are not clear-cut (其中, DBMS和OS在功能上并没有
10、严格界限). Some DBMSs use many access methods of OS (很多DBMS直接利用OS提供的数据存取方法(文件系统), while others create their own file organizations (而另一些DBMS则创建自己的文件组织).,physical level consists of items only OS knows, such as (物理层由只有OS知道的项目构成，例如): how to realise sequencing (如何实现先后顺序), whether fields of records are store
11、d contiguously on disk (记录字段是否连续存放在磁盘上).,2.1 The Architecture of Database Systems,1. External Level 2. Conceptual Level 3. Internal Level 4. Schemas, Mappings, and Instances 5. Data Independence,4. Schemas, Mappings, and Instances,Schema means the description of data (“模式”的含义是“数据的描述” ) For a DB, the
12、re are (一个数据库有): multiple external schemas(or subschemas) (多个外模式或子模式),one conceptual schema(or schema) (一个概念模式或模式),one internal schema (一个内模式),DBMS is responsible for mapping between these three types of schema (DBMS负责实现这三类模式之间的映射). Mapping means transformation of data (映射的含义是数据转换).,4. Schemas, Mapp
13、ings, and Instances,For a DB, there are two types of mapping (数据库中有两类映射): one conceptual/internal mapping ( 一个概念模式/内模式映射);,multiple external/conceptual mappings (多个外模式/概念模式映射).,4. Schemas, Mappings, and Instances,Example of the different levels:,4. Schemas, Mappings, and Instances,It is important to
14、 distinguish between (注意区分下列两个概念) the description of the DB (数据库的描述) and the data in the DB (数据库中的数据). The description of the DB is the DB schema(intension, 内涵) (数据库描述就是数据库模式(又称内涵), which is specified during database design process (它是在数据库设计过程中定义的), and does not change frequently. (且不会频繁变化) The data
15、 in the DB may change frequently (而数据库中的数据通常会频繁变化) The data in DB at a time is called a DB instance(extension, 外延) (某个时刻存放在数据库中的数据被称为一个数据库实例). Many DB instances can correspond to the same DB schema (多个数据库实例能够对应同一个数据库模式).,2.1 The Architecture of Database Systems,1. External Level 2. Conceptual Level
16、3. Internal Level 4. Schemas, Mappings, and Instances 5. Data Independence,5. Data Independence,Logical Data Independence (逻辑数据独立性) refers to immunity(免疫) of external schemas to changes in conceptual schema (指外模式对概念模式变化的免疫:概念模式变化不影响外模式). Conceptual schema changes should not require to modify external schema or application programs (概念模式变化时不必修改外模式和应用程序). This is guaranteed by external/conceptual mappings (逻辑数据独立性由外模式/概念模式映射保证).,5. Data Independence,Physical Data Indepen
