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PERSONNEL table using abstract data types
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Figure 24-1 shows some sample data for this table and the hierarchical column/ field structure created by the abstract data types. Oracle supports abstract data types through a very similar structure, with slightly different SQL syntax. Here is the Oracle CREATE TYPE statement to create the same abstract data structure for names and addresses:
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CREATE TYPE F_NAME M_INIT L_NAME NAME_TYPE AS OBJECT ( VARCHAR(15), CHAR(1), VARCHAR(20));
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CREATE TYPE POST_TYPE AS OBJECT ( MAIN INTEGER, SFX INTEGER); CREATE TYPE STREET CITY STATE POSTCODE ADDR_TYPE AS OBJECT ( VARCHAR(35), VARCHAR(15), CHAR(2), POST_TYPE);
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Oracle calls the abstract data type an object instead of a row type. In fact, the type is functioning as an object class in the usual object-oriented terminology. Extending the object-oriented terminology further, the individual components of an Oracle abstract data type are referred to as attributes (corresponding to the Informix fields described earlier). The ADDR_TYPE type has four attributes in this example. The fourth attribute, POSTCODE, is itself an abstract data type.
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Both Oracle and Informix use the extended dot notation to refer to individual data elements within abstract data types. With nested abstract types, it takes several levels of dot-delimited names to identify an individual data item. The main postal code within the PERSONNEL table is identified as:
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PERSONNEL.ADDRESS.POSTCODE.MAIN
If the table were owned by another user, Sam, the qualified name becomes even longer:
SAM.PERSONNEL.ADDRESS.POSTCODE.MAIN
Informix allows the use of row types to go one step beyond their role as data type templates for individual columns. You can use a row type to define the structure of an entire table. For example, with this row type definition:
CREATE ROW EMPL_NUM NAME ADDRESS TYPE PERS_TYPE ( INTEGER, NAME_TYPE, ADDR_TYPE)
you can define the PERSONNEL table using the row type as a model:
CREATE TABLE PERSONNEL OF TYPE PERS_TYPE;
The columns of this PERSONNEL table will be exactly as they were in the previous CREATE TABLE examples, but now PERSONNEL is a typed table. The most basic use of the typed table capability is to formalize the object structure in the database. Each object class has its own row type, and the typed table that holds objects (rows) of that class is defined in terms of the row type. Beyond this usage, typed tables are also a key component of the Informix notion of table inheritance, described later in the Inheritance section.
Manipulating Abstract Data Types
Unfortunately, structured data types create new complexity for database update statements that must insert or modify their structured data values. Informix Universal Server is fairly liberal in its data type conversion requirements for unnamed row types. The data you assign into a row-type column must simply have the same number of fields, of the same data types. The ROW constructor is used, as shown in previous examples, to assemble individual data items into a row-type value for inserting or updating data.
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For named row types, the requirement is more stringent; the data you assign into a named row-type column must actually have the same named row type. You can achieve this in the INSERT statement by explicitly casting the constructed row value to have the NAME_TYPE data type:
INSERT INTO PERSONNEL VALUES (1234, ROW('John', 'J', 'Jones')::NAME_TYPE, ROW('197 Rose St.', 'Chicago', 'IL', ROW(12345, 6789)));
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The double-colon operator casts the constructed three-field row as a NAME_TYPE row and makes the VALUES clause compatible with the data types of the columns in the table. Oracle uses a slightly different approach to constructing structured data items and inserting them into columns that have abstract data types. When you create an Oracle abstract data type (using the CREATE TYPE statement), Oracle automatically defines a constructor method for the type. You can think of the constructor method as a function that takes as its arguments the individual components of the abstract data type and returns an abstract data type value, with the individual components all packaged together. The constructor is used in the VALUES clause of the INSERT statement to glue the individual data item values together into a structured data value that matches the column definition. Here is an INSERT statement for the PERSONNEL table:
INSERT INTO PERSONNEL VALUES (1234, NAME_TYPE('John', 'J', 'Jones'), ADDR_TYPE('197 Rose St.', 'Chicago', 'IL', POST_TYPE(12345, 6789)));
The constructors (NAME_TYPE, ADDR_TYPE, POST_TYPE) perform the same functions as the ROW constructor does for Informix, and also provide the casting required to ensure strict data type correspondence.
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