Sometimes it is not enough to simply specify the path to an element in your data. What you really want to do is to select some of the data that matches a certain criteria in order to, e. It lets you pick out the data from your database that matches a certain criteria. An XPath predicate allows you to specify selection criteria that will only pick certain elements that match the specified criteria. Predicates are entered as a logical test enclosed by  square brackets , for example:. That means it not only lets you select certain pieces of data — it actually enables you to do all the typical mathematical operations with the data to calculate new values.
Just like formulas in Excel allow you to calculate new values from your existing data, you can do the exact same thing with XPath expressions in MobileTogether. One of the really cool capabilities of XPath is that you can save yourself a lot of typing by doing the multiplication right within the XPath by using parenthesis when you get to the right point in the path, so this shortened version is equivalent to the above expression:.
But you have to use a different symbol for the division. Therefore, the operator for a division in XPath is the word div for a real-number division and idiv for an integer division i.
An introduction to XQuery
Also, it is a good practice — and sometimes essential — to put spaces around your operators to ensure that they are not mistaken for being part of an element or attribute name. This is especially true for the - minus operator. One important difference between XPath and directory paths is that while a directory path expression typically identifies only one particular file or directory, an XPath expression can actually reference more than one data node i. In fact, every XPath expression always returns a sequence of nodes as its result.
Chapter 5. Using XQuery with BDB XML
This is most easily understood if you consider that most databases contain rows of data that have a similar structure, just as most XML documents at their core contain a series of elements that have the same name and share a similar structure. XPath has been created to enable a developer to easily work with such data sets — so being able to address an entire set of nodes at once has huge benefits, as we will see in the next few chapters. At the same time, sometimes it may become necessary to explicitly define sequences in an XPath expression, and you would do that by enumerating the values in the following form:.
Sequences are ordered, so the above sequence is distinctly different from the sequence 3, 5, 2, 4, 1. Sequences can contain nodes from your data, as well as constant numerical values or constant string values. Last, but not least, you can specify ordered sequences of numeric values using the range operator to:. This is equivalent to the sequence 1, 2, 3, 4, 5 — as you can see, this range operator can be used for quickly constructing sequences of numerical values that would be too cumbersome to enumerate by hand as the numbers get larger.
If you have worked with any other programming language in the past, you will recognize the for loop as being a prevalent tool that exists in most modern languages in some form, and it is also an integral part of the XPath expression language. In its simplest form, a for expression in XPath allows you to iterate over a sequence of values or nodes using a temporary loop variable that gets defined directly in the for expression. For example, the expression. Now that we know that any XPath expression can return a sequence of nodes, we can also utilize the for expression to iterate over those nodes and conduct some mathematical operation.
For example, to iterate over all line items in an invoice and calculate the respective amount for that line item, we could use this expression:. Then the previous XPath expression will return the sequence Functions in XPath work exactly like functions in an Excel formula or in most other programming languages: you call a function using its name and passing a set of function parameters in parenthesis.
Introduction to XQuery
For example, to sum up the sequence 3, 9, 14 you would write:. This would give us the result of Apart from the issues actually mentioned, there are also many inconsistencies within and between the working drafts. Current implementations and queries will need major rewrites as the drafts evolve. Update queries including insert queries are specifically not a goal of XQuery version 1.
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Without update queries it is not possible to use XQuery as a complete database interface in the way that SQL is now used for relational database systems. Going by the current drafts, XQuery will not contain full text search facilities like "Find all elements containing a particular word". Considering some of the current application areas of XML, users will have a need for such facilities.
Of course, it is already very hard to give a definition of "word" that satisfies both English and French users, let alone to make it work for Kantonese and every other language as well. These problems may relegate these features to the realm of vendor specific extensions forever. XQuery will use a subset of the yet to be defined XPath 2.
Backward compatibility with XPath 1. However, this does not fit well with several XQuery fundamentals. For example, in XPath 1.
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This means that in many places XQuery is going to be a compromise between the cleanest solution and the one that keeps XPath 1. Though still a lot of work has to be done, XQuery is a very promising initiative in defining the standard for query and retrieval of XML documents and document collections. The majority of XML database vendors, and even some relational database vendors, have developed or announced XQuery implementations. Its mission is to provide superior technology and expertise to the growing market for XML applications and services.
The following standards are currently available:. Previous version based on an article at the French language Wikipedia. From Wikipedia, the free encyclopedia. Functional programming and query language for XML.
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