Extracting Values from XML Documents in PostgreSQL

In this article, we will explore how to extract values from XML documents in PostgreSQL. We will cover the basics of working with XML data, as well as more advanced techniques for extracting specific values.

Introduction

XML (Extensible Markup Language) is a markup language that allows you to store and transport data in a format that is both human-readable and machine-readable. PostgreSQL, being an object-relational database management system, supports the storage and manipulation of XML data.

In this article, we will focus on using XPath expressions to extract specific values from an XML document stored in a PostgreSQL database.

What are XPath Expressions?

XPath (XML Path Language) is a language used to navigate and manipulate XML documents. It allows you to specify a path to the desired data within the XML document, which can then be extracted and processed further.

In the context of PostgreSQL, XPath expressions are used in conjunction with the xpath function to extract values from an XML document.

Creating an XML Document

For this example, we will create an XML document that stores a list of clients:

CREATE TABLE xmltest3(xtxt xml);
INSERT INTO xmltest3 values ('&lt;clients&gt;&lt;client clientId="1435"/&gt;&lt;/clients&gt;');

This creates a table xmltest3 with one column xtxt, which stores the XML data as a string. We then insert an example XML document into the table.

Extracting Values using XPath

To extract values from the XML document, we can use the xpath function in PostgreSQL. This function takes two arguments: the path to the desired data and the XML document itself.

SELECT unnest(xpath('./client /text()', xtxt::xml))::text AS XMLDATA FROM xmltest3;

In this example, we use the XPath expression ./client /text() to extract the value of the clientId attribute from each <client> element in the XML document.

The / character represents the current node (in this case, the root element), and ./client navigates down one level to the <client> element. The final /text() extracts the text content of the element.

However, as we will see later, this approach does not work as expected due to a mistake in the XPath expression.

Correcting the XPath Expression

The issue with the original XPath expression is that it uses ./clients/text(), which would extract the text content from all <clients> elements. Instead, we need to use ./client/@clientId to extract the value of the clientId attribute from each <client> element.

Here’s the corrected query:

with invar as (
  select '&lt;clients&gt;&lt;client clientId="1435"/&gt;&lt;/clients&gt;'::xml as x
) 
select unnest(xpath('/clients/client/@clientId', x))
  from invar;

In this revised example, we use the XPath expression /clients/client/@clientId to extract the value of the clientId attribute from each <client> element.

The resulting output is:

unnest   
--------
1435
(1 row)

This shows that the value of clientId for the first <client> element is indeed 1435.

Extracting Values from Other XML Documents

We can use a similar approach to extract values from other XML documents stored in the database. For example, let’s consider an XML document that stores a list of EN values:

CREATE TABLE xmltest3(xtxt xml);
INSERT INTO xmltest3 values ('&lt;ENList&gt;&lt;EN ENValue="Liquidity"/&gt;&lt;EN ENValue="Treasury"/&gt;&lt;/ENList&gt;');

To extract the values of ENValue from this XML document, we can use the following query:

with invar as (
  select '&lt;ENList&gt;&lt;EN ENValue="Liquidity"/&gt;&lt;EN ENValue="Treasury"/&gt;&lt;/ENList&gt;'::xml as x
) 
select unnest(xpath('/ENList/EN/@ENValue', x))
  from invar;

In this example, we use the XPath expression /ENList/EN/@ENValue to extract the value of the ENValue attribute from each <EN> element.

The resulting output is:

unnest   
-----------
Liquidity
Treasury
(2 rows)

This shows that the values of ENValue for the two <EN> elements are indeed “Liquidity” and “Treasury”.

Conclusion

In this article, we explored how to extract values from XML documents in PostgreSQL using XPath expressions. We covered the basics of working with XML data, as well as more advanced techniques for extracting specific values.

We also corrected a mistake in the original XPath expression, which allowed us to extract the value of clientId from an example XML document.

Finally, we demonstrated how to extract values from other XML documents stored in the database using a similar approach.

Last modified on 2024-05-24