Converting TSQL Output to Nested JSON Format

Learn how to generate `nested JSON` outputs from TSQL queries with practical examples and best practices for SQL Server.
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This video is based on the question https://stackoverflow.com/q/72704042/ asked by the user 'Anthony Horne' ( https://stackoverflow.com/u/1662973/ ) and on the answer https://stackoverflow.com/a/72705403/ provided by the user 'John Cappelletti' ( https://stackoverflow.com/u/1570000/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Converting TSQL Output to Nested JSON Format: A Step-by-Step Guide

In today's data-driven world, JSON (JavaScript Object Notation) has become a popular format for data interchange. SQL Server, especially from version 2016 onwards, includes built-in support for JSON, making it easier to output data in this format. However, one common challenge developers face is outputting data in a structured nested JSON format using TSQL. Let's dive into a specific scenario to understand how to efficiently generate nested JSON results.

The Problem: Outputting Nested JSON

When trying to output JSON data via a TSQL query, you might encounter a scenario where you want a specific format that encapsulates values in a nested structure. Here’s our starting point:

You are trying to combine fields from two different tables (@ header and @ line) and produce JSON output. Your initial query looks something like this:

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The output you’re receiving is as follows:

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However, your desired output is:

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What’s Going Wrong?

The issue lies in the way the JSON is being formatted. Using FOR JSON AUTO creates a simple key-value structure; it does not allow for nested formatting. Fortunately, there are better methods available that adhere to best practices.

The Solution: Generating Nested JSON

To generate a nested JSON structure in TSQL, we can take advantage of the STRING_AGG function and the OPENJSON function, especially when producing values dynamically. Below are techniques you can utilize depending on your SQL Server version.

Method 1: Using STRING_AGG in SQL Server 2017 and Above

For versions 2017 and later, you can utilize the STRING_AGG function along with OPENJSON like so:

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This query will successfully create the structure you desire by aggregating all fields into a single concatenated string.

Method 2: Using XML Path for SQL Server 2016

If you are using SQL Server 2016, you can use the STUFF function in combination with XML Path to achieve a similar output:

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This method utilizes XML processing to concatenate and format the output appropriately, creating a nested JSON structure as required.

Results: Expected Output

Implementing either of the above methods, you should receive the following structured output:

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This output now adheres to the well-formed nested JSON dictated by your specifications.

Conclusion

By carefully utilizing SQL Server's JSON and string aggregation functions, generating a nested JSON output becomes both readable and efficient. This approach not only enhances clarity but also follows best practices, allowing you to harness the full potential of TSQL when working with JSON data. Whether you are using SQL Server 2016 or later, applying these techniques can significantly streamline your data processing tasks.

Feel free to refer back to the methods provided for generating nested JSON directly within your TSQL queries, and embrace the flexibility and efficiency JSON offers in your data operations.