The next example first retrieves all orders with id , and next all orders with id XQJ supports the concept of prepared queries. During the preparation phase, the query is parsed, statically validated, and an optimized execution plan is generated.
This sequence of operations can be a relatively expensive; using XQPreparedExpression objects can improve performance if the same query is executed multiple times. Using prepared queries often implies the use of external variables in your query. The application can bind different values to each of the external variables for different executions. There's also a copy of this example file on the Web. XQuery allows you to access the file directly from either of these locations, using a suitable URL as an argument to its doc function.
Here's an XQuery that simply retrieves and displays the whole document:. This will only work if you are online, of course; and if you're behind a corporate firewall you may have to do some tweaking of your Java configuration to make it work. The file contains a number of sections. That was our first "real" query. If you're familiar with XPath, you might recognize that all the queries so far have been valid XPath expressions.
We've used a couple of functions — current-time and doc — that might be unfamiliar because they are new in XPath 2. Different systems might display this output in different ways.
Technically, the result of this query is a sequence of two element nodes in a tree representation of the source XML document, and there are many ways a system might choose to display such a sequence on the screen.
Stylus Studio gives you the choice of a text view and a tree view: you use the buttons next to the Preview window to switch from one to the other. This example used another function — ends-with — that's new in XPath 2.
We're calling it inside a predicate the expression between the square brackets , which defines a condition that nodes must satisfy in order to be selected. This XPath expression has two parts: a path. The predicate is evaluated once for each selected element; within the predicate, the expression ".
If the path starts with ". Again, this will all be familiar if you already know XPath. XPath is capable of doing some pretty powerful selections, and before we move on to XQuery proper, let's look at a more complex example.
Let's suppose we want to find the titles of all the videos featuring an actor whose first name is Lisa. Each video in the file is represented by a video element like this one:. Many people find that at this level of complexity, XPath syntax gets rather mind-boggling.
In fact, this example is just about stretching XPath to its limits. For this kind of query, and for anything more complicated, XQuery syntax comes into its own. But it's worth remembering that there are many simple things you can do with XPath alone, and that every valid XPath expression is also valid in XQuery.
Note that Stylus Studio also provides a built-in XPath analyzer for visually editing and testing complex XPath expressions, and it supports both version 1. Great, any info on how to call basex from standalone which standalone to use? How does the system know where to go and look for the xquery file? Thanks for your help! You execute BaseX like you execute any other program with your specific operating system. Please take a look at our documentation at docs.
Hi, thanks you so much for your answer. It's interesting, but not exactly what I hoped for. Any idea how is possible to see both of them seperately instead of the sum? Thanks in advance P. David Ennis David Ennis 6, 10 10 silver badges 20 20 bronze badges. Sign up or log in Sign up using Google. Keep in mind that most implementations are experimental and in the technology preview stage.
Although XQuery is still a working draft it already has broad support. There are several applications providing the ability to query using XQuery.
These keywords are commonly used in conjunction to query data and create a result. In technical terms, these expressions are element constructors? Let's start with a simple expression to show you how this works. Note that the expression above is itself not well-formed, and thus is not XML. XQuery represents a departure from this standard.
When you apply the expression above to menu. The above result is a sequence of nodes, and as such is not well-formed XML it has no root element. Hence both the query and the result need not necessarily be XML. A sequence is ordered, so unlike SQL which is set-based, all data returned has a specific order. Returned data can be in the order in which the nodes appear in the source document document order , but you can also manipulate the order.
So how did the data actually get retrieved? First, the document function opens the menu. The curly braces make sure that the XPath expression is evaluated to give a result instead of just showing it as is in the result.
Before we break out of that similarity, let's look at how you can refine the query to select only certain elements by adding WHERE to the mix. When you use the expression above, you actually limit the result to only the desserts from Listing 1 , as shown below:. Note that the id-attribute is actually compared to a string rather than a number.
This is because XQuery uses XPath 2. In the. NET implementation used here, the implicit conversion from string to number doesn't work properly yet. Also only a limited number of the XPath 2.
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