【正文】 rver ? Parallel (multiprocessor) ? Distributed TCGCSU Database Systems Database Users Users are differentiated by the way they expect to interact with the system ? Application programmers – interact with system through DML calls ? Sophisticated users – form requests in a database query language ? Specialized users – write specialized database applications that do not fit into the traditional data processing framework ? Na239。 the database administrator has a good understanding of the enterprise‘s information resources and needs. ? Database administrator39。 customer_name : string。 customer_city : integer。 Logical level: describes data stored in database, and the relationships among the data. ? View level: application programs hide details of data types. Views can also hide information (such as an employee‘s salary) for security purposes. TCGCSU Database Systems Instances and Schemas ? Similar to variables and types in programming languages ? Schema – the logical structure of the database ? Example: The database consists of information about a set of customers and accounts and the relationship between them) ? Analogous to type information of a variable in a program ? Physical schema: database design at the physical level ? Logical schema: database design at the logical level ? Instance – the actual content of the database at a particular point in time ? Analogous to the value of a variable TCGCSU Database Systems Instances and Schemas ? Physical Data Independence – the ability to modify the physical schema without changing the logical schema ? Applications depend on the logical schema ? In general, the interfaces between the various levels and ponents should be well defined so that changes in some parts do not seriously influence others. TCGCSU Database Systems Data Models ? A collection of tools for describing ? Data ? Data relationships ? Data semantics ? Data constraints 描述數(shù)據(jù)、數(shù)據(jù)間聯(lián)系、數(shù)據(jù)語義以及約束的概念工具的集合 ?A data model can be thought of as prising three ponents:數(shù)據(jù)模型由三部分組成： ? Data structure ? Data manipulation ? Constraints of data integrity TCGCSU Database Systems Data Models Data Model: ? Relational model(Table) ? EntityRelationship data model (mainly for database design) ? Objectbased data models (Objectoriented and Objectrelational) ? Semistructured data model (XML) ? Other older models: ? Network model ? Hierarchical model TCGCSU Database Systems Data Manipulation Language (DML) ? Language for accessing and manipulating the data anized by the appropriate data model ? DML also known as query language ? Two classes of languages ? Procedural過程式 – user specifies what data is required and how to get those data ? Declarative聲明式 (nonprocedural) – user specifies what data is required without specifying how to get those data ? SQL is the most widely used query language TCGCSU Database Systems Data Definition Language (DDL) ? Specification notation for defining the database schema Example: create table account ( accountnumber char(10), balance integer) ? DDL piler generates a set of tables stored in a data dictionary ? Data dictionary contains metadata (., data about data) ? Database schema ? Data storage and definition language ? Specifies the storage structure and access methods used ? Integrity constraints ? Domain constraints ? Referential integrity (references constraint in SQL) ? Assertions ? Authorization TCGCSU Database Systems Relational Model ? Example of tabular data in the relational model Attributes TCGCSU Database Systems A Sample Relational Database TCGCSU Database Systems SQL ? SQL: widely used nonprocedural language ? Example: Find the name of the customer with customerid 192837465 select from customer where = ?192837465‘ ? Example: Find the balances of all accounts held by the customer with customerid 192837465 select from depositor, account where = ?192837465‘ and = TCGCSU Database Systems ? Application programs generally access databases through one of ? Language extensions to allow embedded SQL ? Application program interface (., ODBC/JDBC) which allow SQL queries to be sent to a database TCGCSU Database Systems Database Design The process of designing the general structure of the database: ? Logical Design – Deciding on the database schema ( a collection of relation schemas). Database design requires that we find a “good” collection of relation schemas. ? Business decision – What attributes should we record in the database? ? Computer Science decision – What relation schemas should we have and how should the attributes be distributed among the various relation schemas? ? Physical Design – Deciding on the physical layout of the database TCGCSU Database Systems The EntityRelationship Model ? Models an enterprise as a collection of entities and relationships一單位建模為實體與聯(lián)系的集合 ? Entity: a ―thing‖ or ―object‖ in the enterprise that is distinguishable from other objects ? Described by a set of attributes ? Relationship: an association among several entities ? Represented diagrammatically by an entityrelationship diagram: TCGCSU Database Systems ObjectRelational Data Models ? Extend the relational data model by including object orientation and constructs to deal with added data types. ? Allow attributes of tuples to have plex types, including nonatomic values such as nested relations. ? Preserve relational foundations, in particular the declarative access to data, while extending modeling power. ? Provide upward patibilit