The SQL:1999 standard introduced a number of object-relational database features into SQL, chiefly among them structured user-defined types, usually called just structured types. These can be defined either in plain SQL with
CREATE TYPE but also in Java via SQL/JRT. SQL structured types allow single inheritance.
In order to define a custom structure type using Oracle database one could use statements such as these:
CREATE TYPE Person_Type AS OBJECT ( person_title VARCHAR2(10), person_first_name VARCHAR2(20), person_last_name VARCHAR2(20), ) NOT FINAL;
Such structure type can be then used to create a table that would also hold all columns defined in Person_Type:
CREATE TABLE Person_Table OF Person_Type;
Custom structure types support inheritance, which means that one can create another type that inherits from previous.
NOT FINAL statement must be however included in a base structure type definition in order to allow for creation of any other subtypes.
CREATE TYPE Student_Type UNDER Person_Type ( matriculation_number NUMBER(10) );
Student_Type then could be used in order to create a Student_Table which will include all columns defined in Person_Type as well. Primary Key and Constraints should be defined during or after creation of table and cannot be defined inside structure type itself.
CREATE TABLE Student_Table OF Student_Type ( matriculation_number PRIMARY KEY, CONSTRAINT person_title_not_null_constraint NOT NULL (person_title), );
Each custom structure type can also contain other types in order to support more complex structures:
CREATE TYPE Address_Type AS OBJECT ( address_street VARCHAR2(30), address_city VARCHAR2(30), ); CREATE TYPE University AS OBJECT ( university_name VARCHAR2(30), university_address Address_Type );
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