How to Read NULL or Empty Tags from XML in Hive Using explode(XPATH(..))

Discover a simple method to read and handle `NULL` or empty tags in XML within Hive, using the `explode(XPATH(..))` function for better data extraction and manipulation.
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How to Read NULL or Empty Tags from XML in Hive Using explode(XPATH(..))

When working with XML data in Hive, you may encounter challenges while trying to read NULL or empty tags. Specifically, the XPATH() function typically ignores these empty nodes, which can lead to incomplete data extraction. In this post, we will walk through a solution that allows you to effectively read through NULL or empty tags present in XML using the explode(XPATH(..)) function.

Understanding the Problem

Let's take a look at a scenario where we have XML data structured with ParentArray, ParentFieldArray, and various string tags as shown below:

[[See Video to Reveal this Text or Code Snippet]]

In this XML structure, you would like to extract all string tags, including the empty ones. However, using the standard XPATH() function directly results in missing NULL entries, creating an incomplete output.

The Challenge with XPATH()

The issue arises because the XPATH() function returns a NodeList. If the NodeList contains empty nodes, those nodes are not included in the result set. Attempts to concatenate an empty string or use regexp_replace may also lead to errors, as converting empty nodes directly can cause exceptions.

Example of the Error:

When trying to manipulate the XPATH query, you might encounter an error like:

[[See Video to Reveal this Text or Code Snippet]]

A Simple Solution

The key to effectively handling this problem is to replace the empty string tags in the XML with a placeholder value (like NULL) before applying the XPATH() functions. This allows those empty nodes to be recognized and included in the result set without causing errors.

Step-by-Step Implementation

Replace Empty Nodes: Use regexp_replace to modify the XML content, replacing <string></string> and <string/> with <string>NULL</string>.

Extract Values: Then, run your XPATH() queries on the modified XML to get all string values, replacing 'NULL' with actual null values in Hive.

Here’s an example of how to implement this solution:

[[See Video to Reveal this Text or Code Snippet]]

Expected Output

Executing the above SQL will give you the desired output, which will now also include the previously empty values represented as NULL:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By replacing the empty or NULL-like nodes in the XML before processing them with XPATH(), you can successfully read through all tags, ensuring comprehensive data extraction in Hive. This method provides a robust solution to handle XML data with structure that may include missing or empty elements.

Implementing this technique will simplify your data handling and provide more complete results in your Hive queries. Remember, being proactive with data sources, like replacing empty entries, can save significant headaches down the line!