Question

i am new to xquery. I have the following xml document :

<?xml version="1.0" encoding="UTF-8"?>
     <lines>
        <line>
            <id>1</id>
            <par>1</par>
        </line>
        <line>
            <id>2</id>
            <par>1</par>
        </line>
        <line>
            <id>3</id>
            <par>2</par>
        </line>
        <line>
            <id>4</id>
            <par>2</par>
        </line>
        <line>
            <id>5</id>
            <par>1</par>
        </line>
        <line>
            <id>6</id>
            <par>5</par>
        </line>
        <line>
            <id>7</id>
            <par>5</par>
        </line>
        <line>
            <id>8</id>
            <par>5</par>
        </line>
    </lines>

I would like to create a function that would get as input a certain id , and would return all the line elements – descendants of this id , including the one given as input. For example giving as input 1 it would return the line elements with ids 1,2,3,4,5,6,7,8. I know i can go to depth 1 using the following: lines/line[par=id_given], but what if i would like to fetch all descendants?

Answer 1

You have to define a function which traverses the tree recursively. This one does works level-wise:

declare function local:traverse($tree as element(lines), $id as xs:integer*) as element(line)* {
 let $level := $tree//line[par=$id][id!=$id]
 return
  if ($level)
  then ($level, local:traverse($tree, $level/id))
  else ()
};

You can call it using local:traverse(/lines, 0).

But this function will not deal with your example XML, as this is no tree: there is a cycle in the root element which isn’t allowed. You will have to introduce some new unique root id or remove the parent from the root for having a real tree.

<lines>
    <line>
        <id>1</id>
        <par>0</par>
    </line>
    [snip]

You could also change the code above to be able to deal with these self-references: Add the predicate [id!=$id] to the end of the definition of $level, this will exclude self-references. Now your root node is excluded from the results, reinclude it in the function call: (/lines/line[id=1], local:traverse(/lines, 1)).