(3) Result grouping with nested query
The above XML-QL inquiry will not group the result, the author of the same book will appear in different , if the author of the same book is placed in a . To group the results, you can use nested queries. Will change the above examples, you can get the results we want:
WHERE
$ P
in "lib.xml"
$ t </ title>,</p>
<p><Publisher> <Name> Higher Education Press </ Name> </ Publisher> in $ P</p>
<p>Construct <result></p>
<p><title> $ t </ title></p>
<p>Where <author> $ a </ author> in $ P</p>
<p>Construct <Author> $ a </ author></p>
<p></ result></p>
<p>In the above XML-QL statement, first specify the contents of the P variable of <book> element, then determine if the content of the <book> element is included one by one contains <title> element and a <name> "Higher Education Press" <Publisher > Elements, if the conditions are met, the constructs are constructed. The result is composed of the <title> sub-element of the <book> element and a result of a nested query. The result of nested queries consists of all <author> elements in the eligible <book> element.</p>
<p>If the content of the P variable has been specified, it can be used by XML-QL behind. You can also specify the P variable as the content of the <book> element by using the content_as keyword, so that the XML-QL query is simplified to:</p>
<p>WHERE <book></p>
<p><title> $ t </ title></p>
<p><Publisher> <Name> Higher Education Press </ Name> </ Publisher></p>
<p></ book> Content_as $ P in "lib.xml"</p>
<p>Construct <result></p>
<p><title> $ t </ title></p>
<p>Where <author> $ a </ author> in $ P</p>
<p>Construct <Author> $ a </ author></p>
<p></ result></p>
<p>Execute the above two queries, the result is the same:</p>
<p><result></p>
<p><Title> Database System Introduction </ Title></p>
<p><author> <lastname> Sa Master </ lastname> </ author></p>
<p><author> <lastname> Wang Shan </ lastname> </ author></p>
<p></ result></p>
<p>(4) Connecting elements according to value</p>
<p>XML-QL can connect two or more elements including the same value. For example, query all <Article> elements of the author written after 2000:</p>
<p>Where <article></p>
<p><author></p>
<p><firstname> $ f </ firstname></p>
<p><lastname> $ l </ lastname></p>
<p></ author> </ article> content_as $ a in "lib.xml"</p>
<p><book year = $ y></p>
<p><author></p>
<p><firstname> $ f </ firstname></p>
<p><lastname> $ l </ lastname></p>
<p></ author></p>
<p></ book> in "lib.xml"</p>
<p>Y> 2000</p>
<p>Construct <Article> $ a </></p>
<p>In the above query, we used variables f and l to connect. We first specify f and l to <firstname> and <LastName> sub-elements in <author> elements, and specify the A element as the content of the <article> element. It is then judged whether there is a <book> element in <article>, its Year attribute is greater than 2000, and includes <firstname> and <lastname>, respectively, equal to F and L variable <author> element, only in this <book>, <article> element is in line with the conditions of the WHERE statement.</p>
<p>In the above XML-QL, the contents of the construct statement have appeared in the WHERE statement. So, you can use the Element_as statement to avoid such repetitions.</p>
<p>Where <article></p>
<p><author></p>
<p><firstname> $ f </ firstname></p>
<p><lastname> $ l </ lastname></p>
<p></ author></p>
<p></ article> content_as $ a in "lib.xml"</p>
<p><book year = $ y></p>
<p><author></p>
<p><firstname> $ f </ firstname></p>
<p><lastname> $ l </ lastname></p>
<p></ author></p>
<p></ book> Element_as $ E in "lib.xml"</p>
<p>Y> 2000</p>
<p>Construct $ E</p>
<p>(5) Extract data in different XML documents</p>
<p>XML-QL can query multiple XML documents and combine queries. The following example is obtained from Data.xml and TaxPayers.xml two documents to obtain names (<INCOME> Elements) information, and then combine them. And these two elements are linked in <SSN> elements.</p>
<p>WHERE <Person></p>
<p><name> </> Element_as $ n</p>
<p><SSN> $ ssn </></p>
<p></> In data.xml,</p>
<p><TAXPAYER></p>
<p><SSN> $ ssn </></p>
<p><income> </> Element_as $ I</p>
<p></> In turnpayers.xml</p>
<p>Construct <Result> $ N $ I </></p>
<p>In addition to the above method, you can also use the skolem function to implement the above query (see) in the W3C documentation). The specific method is as follows:</p>
<p>{WHERE <Person></p>
<p><name> </> Element_as $ n <ssn> $ ssn </></p>
<p></> In data.xml</p>
<p>Construct <result ID = SSNID ($ SSN)> $ n </></p>
<p>}</p>
<p>{Where <TaxPayer></p>
<p><SSN> $ ssn </></p>
<p><income> </> Element_as $ i</p>
<p></> In turnpayers.xml</p>
<p>Construct <result id = SSNID ($ SSN)> $ I </></p>
<p>}</p></div><div class="text-center mt-3 text-grey">
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