XQuery: Specifying Queries in XML
12.5.2 XQuery: Specifying Queries in XML
XPath allows us to write expressions that select items from a tree-structured XML document. XQuery permits the specification of more general queries on one or more XML documents. The typical form of a query in XQuery is known as a FLWR expression , which stands for the four main clauses of XQuery and has the following form:
FOR <variable bindings to individual nodes (elements)> LET <variable bindings to collections of nodes (elements)> WHERE <qualifier conditions> RETURN <query result specification>
There can be zero or more instances of the FOR clause, as well as of the LET clause in
a single XQuery . The WHERE clause is optional, but can appear at most once, and the RETURN clause must appear exactly once. Let us illustrate these clauses with the fol- lowing simple example of an XQuery .
LET $d := doc(www.company.com/info.xml) FOR $x IN $d/company/project[projectNumber = 5]/projectWorker,
$y IN $d/company/employee WHERE $x/hours gt 20.0 AND $y.ssn = $x.ssn RETURN <res> $y/employeeName/firstName, $y/employeeName/lastName,
$x/hours </res>
1. Variables are prefixed with the $ sign. In the above example, $d , $x , and $y are variables.
2. The LET clause assigns a variable to a particular expression for the rest of the query. In this example, $d is assigned to the document file name. It is possi-
ble to have a query that refers to multiple documents by assigning multiple variables in this way.
3. The FOR clause assigns a variable to range over each of the individual items in a sequence. In our example, the sequences are specified by path expres-
sions. The $x variable ranges over elements that satisfy the path expression $d/company/project[projectNumber = 5]/projectWorker. The $y variable ranges over elements that satisfy the path expression $d/company/employee . Hence, $x ranges over projectWorker elements, whereas $y ranges over employee ele- ments.
4. The WHERE clause specifies additional conditions on the selection of items. In this example, the first condition selects only those projectWorker elements
that satisfy the condition (hours gt 20.0) . The second condition specifies a join condition that combines an employee with a projectWorker only if they have the same ssn value.
5. Finally, the RETURN clause specifies which elements or attributes should be
12.5 XML Languages 435
will return a sequence of elements each containing <firstName, lastName, hours> for employees who work more that 20 hours per week on project
number 5. Figure 12.7 includes some additional examples of queries in XQuery that can be
specified on an XML instance documents that follow the XML schema document in Figure 12.5. The first query retrieves the first and last names of employees who earn more than $70,000. The variable $x is bound to each employeeName element that is a child of an employee element, but only for employee elements that satisfy the quali- fier that their employeeSalary value is greater than $70,000. The result retrieves the firstName and lastName child elements of the selected employeeName elements. The second query is an alternative way of retrieving the same elements retrieved by the first query.
The third query illustrates how a join operation can be performed by using more than one variable. Here, the $x variable is bound to each projectWorker element that is a child of project number 5, whereas the $y variable is bound to each employee ele- ment. The join condition matches ssn values in order to retrieve the employee names. Notice that this is an alternative way of specifying the same query in our ear- lier example, but without the LET clause .
XQuery has very powerful constructs to specify complex queries. In particular, it can specify universal and existential quantifiers in the conditions of a query, aggregate functions, ordering of query results, selection based on position in a sequence, and even conditional branching. Hence, in some ways, it qualifies as a full-fledged pro- gramming language.
This concludes our brief introduction to XQuery . The interested reader is referred to www.w3.org, which contains documents describing the latest standards related to XML and XQuery . The next section briefly discusses some additional languages and protocols related to XML.
1. FOR $x IN
Figure 12.7
doc(www.company.com/info.xml) Some examples of XQuery //employee [employeeSalary gt 70000]/employeeName
queries on XML documents RETURN <res> $x/firstName, $x/lastName </res>
that follow the XML schema file company in Figure 12.5.
2. FOR $x IN doc(www.company.com/info.xml)/company/employee
WHERE $x/employeeSalary gt 70000 RETURN <res> $x/employeeName/firstName, $x/employeeName/lastName </res>
3. FOR $x IN doc(www.company.com/info.xml)/company/project[projectNumber = 5]/projectWorker,
$y IN doc(www.company.com/info.xml)/company/employee
436 Chapter 12 XML: Extensible Markup Language
Parts
» Fundamentals_of_Database_Systems,_6th_Edition
» Characteristics of the Database Approach
» Advantages of Using the DBMS Approach
» A Brief History of Database Applications
» Schemas, Instances, and Database State
» The Three-Schema Architecture
» The Database System Environment
» Centralized and Client/Server Architectures for DBMSs
» Classification of Database Management Systems
» Domains, Attributes, Tuples, and Relations
» Key Constraints and Constraints on NULL Values
» Relational Databases and Relational Database Schemas
» Integrity, Referential Integrity, and Foreign Keys
» Update Operations, Transactions, and Dealing with Constraint Violations
» SQL Data Definition and Data Types
» Specifying Constraints in SQL
» The SELECT-FROM-WHERE Structure of Basic SQL Queries
» Ambiguous Attribute Names, Aliasing, Renaming, and Tuple Variables
» Substring Pattern Matching and Arithmetic Operators
» INSERT, DELETE, and UPDATE Statements in SQL
» Comparisons Involving NULL and Three-Valued Logic
» Nested Queries, Tuples, and Set/Multiset Comparisons
» The EXISTS and UNIQUE Functions in SQL
» Joined Tables in SQL and Outer Joins
» Grouping: The GROUP BY and HAVING Clauses
» Discussion and Summary of SQL Queries
» Specifying General Constraints as Assertions in SQL
» Introduction to Triggers in SQL
» Specification of Views in SQL
» View Implementation, View Update, and Inline Views
» Schema Change Statements in SQL
» Sequences of Operations and the RENAME Operation
» The UNION, INTERSECTION, and MINUS Operations
» The CARTESIAN PRODUCT (CROSS PRODUCT) Operation
» Variations of JOIN: The EQUIJOIN and NATURAL JOIN
» Additional Relational Operations
» Examples of Queries in Relational Algebra
» The Tuple Relational Calculus
» The Domain Relational Calculus
» Using High-Level Conceptual Data Models
» Entity Types, Entity Sets, Keys, and Value Sets
» Relationship Types, Relationship Sets, Roles, and Structural Constraints
» ER Diagrams, Naming Conventions, and Design Issues
» Example of Other Notation: UML Class Diagrams
» Relationship Types of Degree Higher than Two
» Subclasses, Superclasses, and Inheritance
» Constraints on Specialization and Generalization
» Specialization and Generalization Hierarchies
» Modeling of UNION Types Using Categories
» A Sample UNIVERSITY EER Schema, Design Choices, and Formal Definitions
» Data Abstraction, Knowledge Representation, and Ontology Concepts
» ER-to-Relational Mapping Algorithm
» Discussion and Summary of Mapping for ER Model Constructs
» Mapping EER Model Constructs
» The Role of Information Systems
» The Database Design and Implementation Process
» Use of UML Diagrams as an Aid to Database Design Specification 6
» Rational Rose: A UML-Based Design Tool
» Automated Database Design Tools
» Introduction to Object-Oriented Concepts and Features
» Object Identity, and Objects versus Literals
» Complex Type Structures for Objects and Literals
» Encapsulation of Operations and Persistence of Objects
» Type Hierarchies and Inheritance
» Other Object-Oriented Concepts
» Object-Relational Features: Object Database Extensions to SQL
» Overview of the Object Model of ODMG
» Built-in Interfaces and Classes in the Object Model
» Atomic (User-Defined) Objects
» Extents, Keys, and Factory Objects
» The Object Definition Language ODL
» Differences between Conceptual Design of ODB and RDB
» Mapping an EER Schema to an ODB Schema
» Query Results and Path Expressions
» Overview of the C++ Language Binding in the ODMG Standard
» Structured, Semistructured, and Unstructured Data
» XML Hierarchical (Tree) Data Model
» Well-Formed and Valid XML Documents and XML DTD
» XPath: Specifying Path Expressions in XML
» XQuery: Specifying Queries in XML
» Extracting XML Documents from
» Database Programming: Techniques
» Retrieving Single Tuples with Embedded SQL
» Retrieving Multiple Tuples with Embedded SQL Using Cursors
» Specifying Queries at Runtime Using Dynamic SQL
» SQLJ: Embedding SQL Commands in Java
» Retrieving Multiple Tuples in SQLJ Using Iterators
» Database Programming with SQL/CLI Using C
» JDBC: SQL Function Calls for Java Programming
» Database Stored Procedures and SQL/PSM
» PHP Variables, Data Types, and Programming Constructs
» Overview of PHP Database Programming
» Imparting Clear Semantics to Attributes in Relations
» Redundant Information in Tuples and Update Anomalies
» Normal Forms Based on Primary Keys
» General Definitions of Second and Third Normal Forms
» Multivalued Dependency and Fourth Normal Form
» Join Dependencies and Fifth Normal Form
» Inference Rules for Functional Dependencies
» Minimal Sets of Functional Dependencies
» Properties of Relational Decompositions
» Dependency-Preserving Decomposition
» Dependency-Preserving and Nonadditive (Lossless) Join Decomposition into 3NF Schemas
» Problems with NULL Values and Dangling Tuples
» Discussion of Normalization Algorithms and Alternative Relational Designs
» Further Discussion of Multivalued Dependencies and 4NF
» Other Dependencies and Normal Forms
» Memory Hierarchies and Storage Devices
» Hardware Description of Disk Devices
» Magnetic Tape Storage Devices
» Placing File Records on Disk
» Files of Unordered Records (Heap Files)
» Files of Ordered Records (Sorted Files)
» External Hashing for Disk Files
» Hashing Techniques That Allow Dynamic File Expansion
» Other Primary File Organizations
» Parallelizing Disk Access Using RAID Technology
» Types of Single-Level Ordered Indexes
» Some General Issues Concerning Indexing
» Algorithms for External Sorting
» Implementing the SELECT Operation
» Implementing the JOIN Operation
» Algorithms for PROJECT and Set
» Notation for Query Trees and Query Graphs
» Heuristic Optimization of Query Trees
» Catalog Information Used in Cost Functions
» Examples of Cost Functions for SELECT
» Examples of Cost Functions for JOIN
» Example to Illustrate Cost-Based Query Optimization
» Factors That Influence Physical Database Design
» Physical Database Design Decisions
» An Overview of Database Tuning in Relational Systems
» Transactions, Database Items, Read and Write Operations, and DBMS Buffers
» Why Concurrency Control Is Needed
» Transaction and System Concepts
» Desirable Properties of Transactions
» Serial, Nonserial, and Conflict-Serializable Schedules
» Testing for Conflict Serializability of a Schedule
» How Serializability Is Used for Concurrency Control
» View Equivalence and View Serializability
» Types of Locks and System Lock Tables
» Guaranteeing Serializability by Two-Phase Locking
» Dealing with Deadlock and Starvation
» Concurrency Control Based on Timestamp Ordering
» Multiversion Concurrency Control Techniques
» Validation (Optimistic) Concurrency
» Granularity of Data Items and Multiple Granularity Locking
» Using Locks for Concurrency Control in Indexes
» Other Concurrency Control Issues
» Recovery Outline and Categorization of Recovery Algorithms
» Caching (Buffering) of Disk Blocks
» Write-Ahead Logging, Steal/No-Steal, and Force/No-Force
» Transaction Rollback and Cascading Rollback
» NO-UNDO/REDO Recovery Based on Deferred Update
» Recovery Techniques Based on Immediate Update
» The ARIES Recovery Algorithm
» Recovery in Multidatabase Systems
» Introduction to Database Security Issues 1
» Discretionary Access Control Based on Granting and Revoking Privileges
» Mandatory Access Control and Role-Based Access Control for Multilevel Security
» Introduction to Statistical Database Security
» Introduction to Flow Control
» Encryption and Public Key Infrastructures
» Challenges of Database Security
» Distributed Database Concepts 1
» Types of Distributed Database Systems
» Distributed Database Architectures
» Data Replication and Allocation
» Example of Fragmentation, Allocation, and Replication
» Query Processing and Optimization in Distributed Databases
» Overview of Transaction Management in Distributed Databases
» Overview of Concurrency Control and Recovery in Distributed Databases
» Current Trends in Distributed Databases
» Distributed Databases in Oracle 13
» Generalized Model for Active Databases and Oracle Triggers
» Design and Implementation Issues for Active Databases
» Examples of Statement-Level Active Rules
» Time Representation, Calendars, and Time Dimensions
» Incorporating Time in Relational Databases Using Tuple Versioning
» Incorporating Time in Object-Oriented Databases Using Attribute Versioning
» Temporal Querying Constructs and the TSQL2 Language
» Spatial Database Concepts 24
» Multimedia Database Concepts
» Clausal Form and Horn Clauses
» Datalog Programs and Their Safety
» Evaluation of Nonrecursive Datalog Queries
» Introduction to Information Retrieval
» Types of Queries in IR Systems
» Evaluation Measures of Search Relevance
» Web Analysis and Its Relationship to Information Retrieval
» Analyzing the Link Structure of Web Pages
» Approaches to Web Content Analysis
» Trends in Information Retrieval
» Data Mining as a Part of the Knowledge
» Goals of Data Mining and Knowledge Discovery
» Types of Knowledge Discovered during Data Mining
» Market-Basket Model, Support, and Confidence
» Frequent-Pattern (FP) Tree and FP-Growth Algorithm
» Other Types of Association Rules
» Approaches to Other Data Mining Problems
» Commercial Data Mining Tools
» Data Modeling for Data Warehouses
» Difficulties of Implementing Data Warehouses
» Grouping, Aggregation, and Database Modification in QBE
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