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Part VI:
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number of arguments with identical data types. In the previous example, there would be three CREATE FUNCTION definitions like this:
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/* Calculates target wages for a technician */ CREATE FUNCTION GET_TGT_WAGES(PERSON TECH_TYPE) RETURNS DECIMAL(9,2) AS RETURN (PERSON.WAGE_RATE * 40 * 52) END FUNCTION; /* Calculates target wages for a manager */ CREATE FUNCTION GET_TGT_WAGES(PERSON MGR_TYPE) RETURNS DECIMAL(9,2) AS RETURN (PERSON.SALARY + PERSON.BONUS) END FUNCTION; /* Calculates target wages for an engineer */ CREATE FUNCTION GET_TGT_WAGES(PERSON ENGR_TYPE) RETURNS DECIMAL(9,2) AS RETURN (PERSON.SALARY) END FUNCTION;
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With these definitions in place, you can invoke the GET_TGT_WAGES() function and pass it a row from the ENGINEERS, MANAGERS, or TECHNICIANS table. The DBMS automatically figures out which of the functions to use and returns the appropriate calculated value. Stored procedures are made even more valuable for typed tables through Informix Universal Server s substitutability feature. If you call a stored procedure whose argument is a row type and pass it one of the rows from a typed table, Informix will first search for a stored procedure with the appropriate name whose argument data type is an exact match. For example, if you call a GET_LNAME() stored procedure to extract the last name from a TECH_TYPE row (probably from the TECHNICIANS table), Informix searches for a procedure written to process TECH_TYPE data. But if Informix doesn t find such a stored procedure, it does not immediately return with an error. Instead, it searches upwards in the type hierarchy, trying to find a procedure with the same name that is defined for a supertype of TECH_TYPE. If there is a GET_LNAME() stored procedure defined for the ENGR_TYPE type, Informix will execute that stored procedure to obtain the required information. If not, it will continue up the hierarchy, looking for a GET_LNAME() stored procedure defined for the PERS_TYPE type. Thus, substitutability means that you can define stored procedures (methods) for the highest-level type in the hierarchy to which they apply. The stored procedures are automatically available to process all subtypes of that type. (That is, all subclasses inherit the method from the class.)
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As mentioned at the beginning of this chapter, the largest area of SQL expansion in the SQL:1999 standard was object-relational support. New statements, clauses, and expressions were added to the specification of the SQL in these areas: User-defined data types Composite (abstract) data types Array values
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24:
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Overloaded (polymorphic) stored procedures Row constructors and table constructors supporting abstract types Row-valued and table-valued expressions supporting abstract types The SQL standard object extensions don t exactly match any of the major commercial object-relational DBMS products in their specifics, but the underlying concepts are the same as those illustrated in the earlier sections for specific products. It s likely that this area of SQL will follow the pattern of others with respect to the standard. Slowly, over a series of major releases, the major DBMS vendors are providing support for the SQL standard syntax where it can be added in parallel to their own, well-established proprietary syntax. This process is well under way for SQL object support. For the next several years, the objectrelational capabilities that matter for real-world implementations will continue to be a mixture of standard features augmented with vendor-proprietary capabilities.
Summary
Object-oriented databases will likely play an increasing role in specialized market segments such as engineering design, compound document processing, and GUIs. They have not been widely adopted for mainstream enterprise data processing applications. However, hybrid object-relational databases are being offered by some of the leading enterprise DBMS vendors: The object-relational databases significantly extend the SQL and stored procedure languages with object-oriented statements, structures, and capabilities. Common object-relational structures include abstract/structured data types, tables within tables, and explicit support for object identifiers. These capabilities stretch the simple relational model a great deal and tend to add complexity for casual or ad hoc users. The object-relational extensions added by the various DBMS vendors are highly proprietary. There are significant conceptual differences in the approaches as well as differences in implementation approach. Object-relational capabilities are particularly well suited for more complex data models, where the overall design of the database may be simpler, even though individual tables/objects are more complex. Object-relational capabilities are a major focus of the SQL standards efforts, and more relational databases are likely to incorporate them in the future.
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