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ODL Conception

Yu Miao. ODL Conception. Object-oriented Conception. 功能强大的类型系统 类的一个特点：可以拥有方法 对象标识 （ object identity ） 每个对象都有一个与其值无关的标识 继承 使面向 对象中类的关系层次化. Object-oriented Conception. 类型系统 面向对象的 语言 允许 用户 创建自定义类型，包括： 记录结构 集合类型 引用类型： 对类型 T 的一个引用 是这样的一个类型，它可以用来指向该类型的一个特定的值。（ C/C++ 中的指针 ） 对象标识

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ODL Conception

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  1. Yu Miao ODL Conception

  2. Object-oriented Conception • 功能强大的类型系统 • 类的一个特点：可以拥有方法 • 对象标识 （object identity） 每个对象都有一个与其值无关的标识 • 继承 使面向 对象中类的关系层次化

  3. Object-oriented Conception • 类型系统 • 面向对象的语言允许用户创建自定义类型，包括： • 记录结构 • 集合类型 • 引用类型： 对类型T的一个引用 是这样的一个类型，它可以用来指向该类型的一个特定的值。（C/C++中的指针） • 对象标识 • 对象都有对象标识（object identity, OID）， 任何对象有且只有一个与其它对象不同的OID。

  4. ODL 简介 • ODL（object definition language）是一种用面向对象的术语描述数据库结构的标准化语言。 在设计ODL类时，要描述三类特性： • 属性（attribute）：它是与对象相关的值 • 联系(relationship)：它是当前对象与其它对象的连接 • 方法（method）：它是可以应用于类的对象的函数

  5. ODL 中的属性 • 在ODL中，属性不必为整型、字符串等简单类型，这一点与E/R模型不同。 Class movie { Attribute string title; Attribute integer year; Attribute integer length; Attribute enum file {color, blackAndWhite} fileType; } • 一个movie类的对象可以被视为一个拥有四个分量的记录或元组，每个分量都对应一个属性。以上的属性都是原子属性，属性也可以是非原子性的，例如： Attribute structAddr {string street, string city} address;

  6. ODL中的联系 • Relationship Set<Star> stars; • Inverse Movie::stars

  7. Methods in ODL • As in other object-oriented languages, a method is a piece of executable code that might be applied to the objects of the class. • In ODL, we can declare the name of the methods associated with a class and the input/output types of those methods. These declarations, called signatures, are like functions in C or C++.

  8. Types in ODL • ODL offers the database designer a type system similar to that found in C or other conventional programming languages. • A type system is built from a basis of types that are defined by themselves and certain recursive rules whereby complex types are built from sample types.

  9. Types in ODL • In ODL, the basis consists of: • Atomic types: integer, float …. • Class names, such as Movie, or Star

  10. Types in ODL • These basic types are combined into structured types using the following type constructors: • Set. • Bag. If T is any type, then Bag<T> denotes the type whose value are finite bag or multisets of elements of type T. A bag allows an element to appear more than once. For example, {1, 2,1} is a bag but not a set. • List. List<T> denote the type whose values are finite list of zero or more elements of type T. as a special case, the type string is a shorthand for the type List<char>. • Array. Array<T, i> denotes the type whose elements are arrays of i elements of type T. for example, Array<char, 10> denotes character strings of length 10. • Dictionary.

  11. Additional Concept in ODL • Representing multiway relationships. • Subclasses and inheritance. • Keys, which are optional in ODL • Extents, the set of objects of a given class that exist in a database. These are the ODL equivalent of entity sets or relations and must not be confused with the class itself, which is a schema.

  12. Multiway relationship in ODL • ODL supports only binary relationship. There is a trick to replace a multiway relationship by several binary, many-one relationships.

  13. Subclass in ODL Class cartoon extends Movie { Relationship Set<Star> voices; } A subclass inherits all the properties of its superclass. Thus, each cartoon object has attribute title, year, length and filmType inherited from Movie, and it inherits relationships stars and ownedBy from Movie in addition to its own relationship voices.

  14. Multiple inheritance in ODL • A principle of object-oriented systems is that object belongs to one and only one class. • In ODL, we may follow the key word extend by several classes, separated by colons(:). Thus we may declare the fourth class by: Class CartoonMurderMystery Extend MurderMystery : Cartoon;

  15. Multiple inheritance in ODL • What happen if two super have same property names? • Some possible approaches: • Disallow multiple inheritance altogether. • Indicate which of the candidate of the definitions of the property applies to the subclasss. • Give a new name in the subclass for one of the identically named properties in the superclasses.

  16. Extent • we need distinguish the class definition itself from the set of objects of that class that exists at a given time. • In ODL, the distinction is made explicit by giving the class and its extent, or set of existing objects, different names. Class Movie extent(Movies)

  17. Keys in ODL • the use of keys is optional in ODL. • objects have a unique object identity(OID). • The ODL standard also allow properties other than attribute to appear in keys.

  18. 原子类型的值，是特点还是不足？ ODL允许使用结构类型，它使得关系模型显得碍手碍脚。但是，现状是关系型数据库至今仍在市场上占主导地位。一个重要的原因是， 这种模型的简单性使得其查询语言的功能十分强大，特别是SQL， 它是现在数据库中的查询语言标准。

  19. 对象关系模型 • 关系模型及ODL这样的面向对象 的模型是DBMS的两个重要模型。长久以来，主流DBMS模型一直属于关系模型。上世纪90年代，面向对象模型有了一定发展，但随即又消失了。改变是关系型模型的数据库引入了许多面向对象模型中的概念。因此出现了对象关系模型（object-relational）

  20. 关系加入以下特点后就成为对象关系： • 结构类型的属性 • 方法 • 元组的标识符 • 引用

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