Object-Oriented SQL in Java

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Object-Oriented SQL
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It seemed that the SQL lectures were the most difficult ones for students, particularly when dealing with recursive queries and those that use down-casting operators. Before describing some of the problems in detail that the students had, let us summarize the basics of the object-oriented SQL that were taught in the subject. Basically we used the SQL language designed by Won Kim in the context of the ORION system. This SQL was chosen for several reasons. The main reason was that ORION SQL provides "richer" facilities to express a variety of queries, including: Queries over aggregation hierarchies Queries over inheritance hierarchies Recursive queries Queries with methods We began teaching how to express simple queries and then extended these to include more advanced selections, such as queries over aggregation hierarchies. This illustrated the complexity OOSQL queries to students, and it highlighted the differences between those queries and relational queries. We were able to clearly explain the different extensions of the relational basic SQL queries to deal with the complex queries (that is, those queries that deal with object-oriented concepts, such as inheritance and aggregation hierarchies, methods, and recursion). Another reason we choose the ORION SQL query language was that it provides a simple syntax (similar to the relational one) and is not specific to any particular system. It extends the relational SQL to allow parts of predicates to include simple or complex paths. The gradual extension of the basic SQL syntax to deal with complex queries (e.g., queries on aggregation and inheritance hierarchies), in addition to examples, makes the concepts easier for students to understand.
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Despite the details given to students about OOSQL, including examples and even articles, several of them struggled to express complex queries in OOSQL. Here, we summarize some of the problems: Complex paths. Queries that are expressed on aggregation hierarchies are based on paths. An example of a complex path query on the schema of Figure 12.2 is: "Find
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dence is in Rome and who drives a Ferrari" (Q1). This query can be expressed in several equivalent ways; however, the most suitable is one that fully uses object-oriented concepts. Even though students learned how to express such types of queries in an object-oriented way (using a specific syntax), they still kept using the "relational way" of expressing paths as a set of joins. The relational way of expressing query Q, is as follows: :C from Company :C, Division :D, Employee :E :D is-in :C division and :E is-in :D staff and :E drives manufacturer name = "Ferrari" and :E residence = "Rome" In this SQL query, students introduced several variables to ensure that joins between the different classes could be made. In this case, abstractions (such as Company, Division, and Employee) are seen as relations and therefore the aggregation relationships are used as foreign keys to join them. During lectures and tutorials, we had demonstrated that the navigation through different classes did not require the introduction of variables. We used the following example to illustrate the navigation aspects of OOSQL queries. select where :C from Company :C, E: is-in :C divisions staff :E drives manufacturer name = "Ferrari" and :E residence = "Rome" select where
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Multivalued attributes. Such attributes pose some problems because a path variable involving such attributes requires the use of quantifiers, such as the use of the operators exist or each. Referring again to the example of Figure 12.2, the attribute division of Company is multivalued. Therefore, the specification of such an attribute with a path requires the use of a quantifier beforehand to state explicitly that either every division or at least one division needs to be considered in the predicate. The use of such quantifiers within path variables, combined with the use of variables, has been taught both by using the syntax as well as examples. Student comprehension improved after several exercises were completed using such quantifiers. Down-casting operator. As in previous examples, traversing different inheritance hierarchies within a schema can be done by introducing variables. For instance, if the query aims at "finding all employees who drive a car with a turbo engine of more that 100HP", then this "crosses" three different hierarchies: one with Employee as a root, the other one with Automobile as a root, and finally the last one with TurboEngine as a root. Therefore, instead of using the path that joins the three classes, students used the different variables to "control" the scope the predicate, as shown:
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