Query processing is the entire process of translating a user query into a (procedural) query execution plan for a system and then executing that plan. This process requires a variety of (query) optimization phases.
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XPath
XPath (pronounced "ex-path") is an expression language to find information in an XML document. It is defined by the World Wide Web Consortium and used in many applications that support XML, such as Web browsers and many programming languages. It is used to traverse the elements and attributes of an XML document by using a tree representation and a set of expressions. XPath is a major part of the XSLT standard and it is required knowledge to work with XSLT documents.
The XPath 1.0 specification became a W3C Recommendation in 1999 and is widely implemented and used, either on its own or embedded within other W3C standards such as XSLT, XProc, XML Schema, and XForms. It is also used in a number of XML processing libraries and tools as an alternative to CSS Selectors. XPath 2.0 became a Recommendation in 2007 and adds support for the XDM data model with a richer type system.
XQuery
XQuery is an expressive functional programming language that queries and transforms collections of structured data, usually in the form of XML documents. It can also be used to query and manipulate data from other sources that can be treated as XML, such as relational databases and office documents. It is developed by the W3C XML Query Working Group and coordinates its work with the XSLT Working Group. XQuery uses the same syntax as XPath, and a large part of its functionality is identical to that of XSLT. However, a number of differences are notable. Unlike XSLT, XQuery does not allow XPath expressions to be nested in XQuery instructions, but both languages share the same set of primitive data types: integers and floating-point numbers; boolean values true and false; string literals, and a number of different date and time data types.
The 'X' in 'XQuery' stands for 'Extensible Markup Language', but it's designed to access a lot more than just files in that format. It's intended to be the equivalence of SQL for XML, and it can handle things like extracting information from a Web document and transforming it into XHTML for presentation. The heart of XQuery is the FLWOR expression, which looks rather like an SQL Select statement.
SQL
SQL is a declarative query language that provides users with a simple, easy-to-use, and intuitive way to interact with relational data. It allows database users to select, insert, update and delete data. It also allows for more complex operations, such as joining data sets. SQL is used as a standard in many database management systems (DBMS). DBMSs that support SQL typically have a variety of different query processing algorithms and mechanisms to enable efficient operation.
The simplest form of a SQL query is a single-scan query, which requires the database server to access each record in a table (relation) and verify that it meets a particular criterion. For example, a query could request the database to list all crime hot spots within a circle of 10 miles from downtown Minneapolis. For more details please visit Faraz Berjis - Health Coach
The SQL query processor converts the user's high-level query into low-level expressions at the file system's physical level, optimizes the query for its execution on a DBMS, and executes the query to obtain the actual result. To achieve this, it uses accurate metrics such as cardinality and number of result tuples to calculate an estimate for the query's cost and heuristics to determine the best join type. It also determines which tables will be accessed, and how they will be accessed, based on available indexes and the size of involved relations.