XQuery Update Extension

Introduction

eXist-db provides an extension to XQuery for updating nodes in the database. The extension makes the following operations possible using a simple syntax: insert, delete, replace, update value, and rename.

Important Considerations

Relationship to W3C Recommendation

This extension was created well before the W3C working group created the XQuery Update Facility 1.0 recommendation, and it differs substantially from the recommendation. However, it remains perfectly functional and is still the primary method (besides XUpdate) for updating nodes.

Persistent Document Updates

The XQuery update extension has been designed around updating persistent documents stored in the database. It is not suitable for updating temporary document fragments constructed within an query, i.e., you can't use it to modify the results returned by a query. For example, the following query has no visible effect because it operates on an XML node constructed in-memory:

let $node := <root><a/></root> return update insert <b/> into $node/a

Since $node is an in-memory constructed element, rather than a node stored in the database, the query has no effect and simply returns the empty element.

Update Syntax

All update statements start with the keyword "update", followed by an update action. Available actions are: "insert", "delete", "replace", "value" and "rename". The return type of the expression is empty().

An update statement may occur at any position within the XQuery main code or a function body.

When using an update within the return clause of a FLWOR expression, be cautious when deleting or replacing nodes that are still being used by enclosing code. This is because a delete or replace will be processed immediately, and the deleted or replaced node will no longer be available to the query. Such actions can corrupt the database. For example, the following expression will throw the database into an inconsistent state if //address returns more than one node:

for $address in //address return update delete //address

An unsafe update

However, an expression like the following is safe as it only modifies the current iteration variable (note that the following example deletes $address (the current iteration variable) rather than //address (all addresses in the database, including others that have not been deleted yet):

for $address in //address return update delete $address

A safe update

Aside from this caveat, eXist-db's XQuery update extension is safe to use.

Insert

update insert expr ( into | following | preceding )
                exprSingle

Inserts the content sequence specified in expr into the element node passed via exprSingle. exprSingle and expr should evaluate to a node set. If exprSingle contains more than one element node, the modification will be applied to each of the nodes. The position of the insertion is determined by the keywords "into", "following" or "preceding":

into: The content is appended after the last child node of the specified elements.
following: The content is inserted immediately after the node specified in exprSingle.
preceding: The content is inserted before the node specified in exprSingle.

update insert <email type="office">andrew@gmail.com</email> into //address[fname="Andrew"]

Insert Example: Element

update insert attribute type {'permanent'} into //address[fname="Andrew"]

Insert Example: Attribute

Replace

update replace expr with exprSingle

Replaces the nodes returned by expr with the nodes in exprSingle. expr may evaluate to a single element, attribute or text node. If it is an element, exprSingle should contain a single element node as well. If it is an attribute or text node, the value of the attribute or the text node is set to the concatenated string values of all nodes in exprSingle. expr cannot be the root element of a document.

update replace //fname[. = "Andrew"] with <fname>Andy</fname>

Replace Example

Value

update value expr with exprSingle

Updates the content of all nodes in expr with the items in exprSingle. If expr is an attribute or text node, its value will be set to the concatenated string value of all items in exprSingle.

update value //fname[. = "Andrew"] with 'Andy'

Value Example

Delete

update delete expr

Removes all nodes in expr from their document. expr cannot be the root element of a document.

for $city in //address/city return update delete $city

Delete Example

Rename

update rename expr as exprSingle

Renames the nodes in expr using the string value of the first item in exprSingle as the new name of the node. expr should evaluate to a set of elements or attributes. expr cannot be the root element of a document.

for $city in //address/city return update rename $city as 'locale'

Rename Example