Queries over Objects in Java

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Queries over Objects
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One of the advantages of using object REFs and collections of object REFs in an object-relational schema design is that REFs allow a more direct representation of objectoriented conceptual schemas. As a result, queries can make use of REFs to navigate through object relationships instead of explicitly specifying join conditions. As an example, suppose that we want to return the name of the department of a club's advisor. The following select statement uses object references to traverse through the single-valued relationships that exist from a club to its advisor and from an advisor to the department in which the advisor works:
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select from where c.Advisor.WorksIn.Name clubs c c.name = "Epsilon Sigma";
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This query returns a string value representing the name of the appropriate department. Queries that return objects must be written either to return the object REF or to return the tuple that represents the object type. The DEREF function is provided to access the actual type of an object reference. As an example, the following query retrieves the code and name of the department in which Pablo is a major:
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DECLARE d department; BEGIN select DEREF(p.Major) into d from persons p where p.FirstName = "Pablo"; END;
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In this query, p.Major returns a REF to a department object. DEREF(p.Major) returns a tuple of type department, which contains the Code and Name attributes as defined in Listing 2.1.
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Comparison to Other Systems
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As part of our study, we performed comparisons of the mappings described in the previous sections to:
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Mapping UML Diagrams to Object-Relational Schemas in Oracle 8
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The relational model A different object-relational model An object-oriented database model Due to its object-oriented nature, UML obviously provides a smoother mapping to an object-relational model than to a traditional relational model. Object tables, nested tables, VARRAYS, and object references have provided a more natural representation of UML data than relational tables. The object extensions to SQL also provided a more concise expression of queries compared to traditional relational queries. Class definitions and user-defined types, however, require the generation of the appropriate methods and triggers that subsequently can be used to access objects and complex types as well as to check the appropriate constraints associated with such types. As a result, the translation of conceptual schemas from a model such as UML to an object-relational schema is more complex than traditional ER-to-relational mapping techniques. The mappings that we have defined not only identify the object types, relational tables, and object tables of the target schema, but also define the manner in which the methods and triggers associated with types and tables should be used. PostgreSQL was the first database management system that utilized object-relational database concepts. The system is public-domain software that supports a subset of SQL92 and some extended features of object-relational systems. Illustra Information Technology (a wholly owned subsidiary of Informix Software, Inc.) also provides a commercial object-relational database system called Data Blade, which was originally based on Postgres (Stonebraker 1996). Since Data Blade was not freely available at the time of this study, PostgreSQL is used for comparison in this subsection. Although PostgreSQL also has a CREATE TYPE statement, the way it is used is different from the Oracle 8 CREATE TYPE statement. The approach in Oracle 8 for defining object types is to create a composite type with its associated member functions (like creating a class in C++) that can be used as attribute types or to create object tables. In PostgreSQL, the CREATE TYPE statement will create a new base (primitive) type that is stored as a TEXT type internally, except for the base type that is used as an array. The new base type, which is referred to as a user-defined type, can have a name at most 15 characters long. A PostgreSQL user-defined type must have input and output functions. These functions are used to determine how the new type appears in string form for input by and output to the user, and how the new type is stored in memory. Contrary to Oracle 8, the PostgreSQL user-defined type cannot specify any encapsulated member functions. Global functions are used to access and manipulate the type. PostgreSQL also does not have strong procedural language support as with Oracle's PL/SQL. As a result, PostgreSQL requires the use of native calls implementation for most of the PostgreSQL functions. A table in PostgreSQL is the same as an object table in Oracle 8. Every instance in PostgreSQL has a unique object identifier, which is stored implicitly in a special attribute called aid. A PostgreSQL table can also be used as a user-defined type for arguments in PostgreSQL functions or as an attribute type. Because of the PostgreSQL table properties, a UML class would be mapped using a PostgreSQL table. Forward relationships between objects are implemented using the OID built-in type (or an array of OIDs for multivalued relationships). A PostgreSQL OID can be viewed as a generic OID that stores only the persistent object logical address. To retrieve the object instance that owns the OID, an SQL join operation has to be performed.
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