Adbms 17 object query language

Adbms 17 object query language

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  Advance Database ManagementSystems:17 Object Query Language Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
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  OQL: OBJECT-ORIENTED QUERY LANGUAGE •OQL is a query language designed to operate on databases described in ODL. • • Tries to bring some concepts from the relational model tothe ODBMs • • E.g., the SELECT statement, joins, aggregation, etc. • • Reference of class properties (attributes, relationships, and methods) using: – • Dot notation (p.a), or – • Arrow notation (p->a) • • In OQL both notations are equivalent
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  Slide 21- 3 ObjectQuery Language • OQL is DMG’s query language • OQL works closely with programming languages such as C++ • Embedded OQL statements return objects that are compatible with the type system of the host language • OQL’s syntax is similar to SQL with additional features for objects
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  Slide 21- 4 SimpleOQL Queries • Basic syntax: select…from…where… – SELECT d.name – FROM d in departments – WHERE d.college = ‘Engineering’; • An entry point to the database is needed for each query • An extent name (e.g., departments in the above example) may serve as an entry point
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  Slide 21- 5 IteratorVariables • Iterator variables are defined whenever a collection is referenced in an OQL query • Iterator d in the previous example serves as an iterator and ranges over each object in the collection • Syntactical options for specifying an iterator: – d in departments – departments d – departments as d
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  Slide 21- 6 DataType of Query Results • The data type of a query result can be any type defined in the ODMG model • A query does not have to follow the select…from…where… format • A persistent name on its own can serve as a query whose result is a reference to the persistent object. For example, – departments; whose type is set<Departments>
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  Slide 21- 7 PathExpressions • A path expression is used to specify a path to attributes and objects in an entry point • A path expression starts at a persistent object name (or its iterator variable) • The name will be followed by zero or more dot connected relationship or attribute names – E.g., departments.chair;
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  Slide 21- 8 Viewsas Named Objects • The define keyword in OQL is used to specify an identifier for a named query • The name should be unique; if not, the results will replace an existing named query • Once a query definition is created, it will persist until deleted or redefined • A view definition can include parameters
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  Slide 21- 9 AnExample of OQL View • A view to include students in a department who have a minor: define has_minor(dept_name) as select s from s in students where s.minor_in.dname=dept_name • has_minor can now be used in queries
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  OQL: EXAMPLE QUERIES
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  Assignment • What isOQL. Give Examples to explain its functionalities.