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Overview

Description

To explain the concept of inheritance, a table is expressed as a class, a column is expressed as an attribute, and a type is expressed as a domain.

Classes in CUBRID database can have class hierarchy. Attributes and methods can be inherited through such hierarchy. As shown in the previous section, you can create a Manager class by inheriting attributes from an Employee class. The Manager class is called the sub class of the Employee class, and the Employee class is called the super class of the Manager class. Inheritance can simplify class creation by reusing the existing class hierarchy.

CUBRID allows multiple inheritances, which means that a class can inherit attributes and methods from more than one super class. However, inheritance can cause conflicts when an attribute or method of the super class is added or deleted.

Such conflict occurs in multiple inheritance if there are attributes or methods with the same name in different super classes. For example, if it is likely that a class inherits attributes of the same name and type from more than one super class, you must specify the attributes to be inherited. In such a case, if the inherited super class is deleted, a new attribute of the same name and type must be inherited from another super class. In most cases, the database system resolves such problems automatically. However, if you don't like the way that the system resolves a problem, you can resolve it manually by using the INHERIT clause.

When attributes are inherited from more than one super class, it is possible that their names are to be the same, while their domains are different. For example, two super classes may have the same attribute, whose domain is a class. In this case, a sub class automatically inherits attributes with more specialized (a lower in the class hierarchy) domains. If such conflict occurs between basic data types (e.g. STRING or INTEGER) provided by the system, inheritance fails.

Conflicts during inheritance and their resolutions will be covered in the Resolving Class Conflicts section.

Remark

The following cautions must be observed during inheritance:

  • The class name must be unique in the database. An error occurs if you create a class that inherits another class that does not exist.
  • The name of a method/attribute must be unique within a class. The name cannot contain spaces, and cannot be a reserved keyword of CUBRID. Alphabets as well as '_', '#', '%' are allowed in the class name, but the first character cannot be '_'. Class names are not case-sensitive. A class name will be stored in the system after being converted to lowercase characters.

Note A super class name can begin with the user name so that the owner of the class can be easily identified.