Solving the Athena unnest Problem for Nested Array String Columns

Learn how to effectively use `Athena unnest` function to handle nested array string columns in Amazon Athena and extract values across multiple rows.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Athena unnest for nested Array string column

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Solving the Athena unnest Problem for Nested Array String Columns

Amazon Athena is a powerful tool for querying data directly in Amazon S3 using SQL-like queries. However, working with nested data structures, especially when they are represented as strings, can present some unique challenges. In this post, we will dive into a common problem encountered by users: extracting values from nested JSON-like string columns in Athena, and we will provide an effective solution.

Understanding the Problem

Imagine you have a column in your Athena database that contains data structured as a string that looks something like this:

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

In this example, each entry in the array contains keys and values, including the outer_key, whose values you want to split across multiple rows in your query output.

For instance, if you desire the output in a format similar to the following:

idouter_key1outer_value1outer_value1the challenge lies in transforming the nested structure effectively into a tabular format.

The Solution

Step 1: Determine the Data Type

First, you need to confirm whether your nested data is actually a valid JSON string or merely a standard string. This distinction affects the approach you will take to extract the necessary values.

Step 2: If It's a Valid JSON String

If the data in your column is a valid JSON string, you can use the following SQL query to extract the outer_key values:

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

In the query above:

We create a common table expression (CTE) with the sample JSON data.

Using UNNEST, we break down the nested array and extract the outer_key values.

Output:

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

Step 3: If It's Just a String

If your data is not valid JSON but a standard string format, you would need to apply regular expressions to extract the desired values. Here is an example of how you could do this:

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

In this situation:

We use regexp_extract_all to find all matches for the outer_key pattern in the string.

Output:

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

Conclusion

Working with nested data structures in Athena can be complex, but with the right approach, you can extract the values you need. Whether your data is structured as valid JSON or as a regular string, both methods outlined above will help you retrieve outer_key values across multiple rows.

Feel free to implement these queries in your AWS Athena environment and don't hesitate to tweak them further according to your specific data format.

Happy querying!