Fixing JSON Updates in SQL: Modifying Arrays without Losing Data

Learn how to properly update JSON in SQL Server when dealing with arrays of objects to prevent data loss.
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Fixing JSON Updates in SQL: Modifying Arrays without Losing Data

Introduction

Have you ever faced the challenge of updating a JSON column in SQL that contains an array of objects? If you've tried modifying a specific property within that array and ended up replacing the entire array with just the object you intended to change, you’re not alone. This common issue arises due to how SQL Server handles JSON data, especially when using functions like OPENJSON. In this post, we’ll explore the problem and provide a step-by-step solution to modify your JSON data effectively.

The Problem

Consider a JSON column structured as follows:

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

In this array, you need to update the LeftTime property that currently holds a null value, replacing it with the current UTC date using the GETUTCDATE() function. If you’ve tried doing this with a straightforward UPDATE query, you might have found that it only updates the specific object and ends up discarding the other entries in the array.

Understanding the Mechanism

The underlying issue is about how SQL Server parses and manipulates JSON data. The OPENJSON() function creates a result set from the JSON array but does not inherently support modifying individual elements while preserving the entire structure. When you use JSON_MODIFY(), it may not work with wildcard paths as you might expect, leading to overwriting the entire JSON array with the modified object.

Solution Overview

To avoid data loss while updating the LeftTime property, we will follow these steps:

Use OPENJSON() to read the array and its elements.

Modify the LeftTime property where it is null using COALESCE(), which allows for fallback values.

Reconstruct the JSON array back into the original column.

Step-by-Step Implementation

1. Initial Setup

Let's create a temporary table to hold your JSON data (similar to the original structure you might have):

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

2. The Update Statement

The SQL statement below will effectively update only the null values in the LeftTime property without losing the rest of the data:

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

3. Resulting Data

After executing the above update, the JSON column would reflect the changes as follows:

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

The LeftTime has now been updated to the current date while preserving all other entries in the array.

Conclusion

Updating JSON arrays in SQL can be tricky, but with a proper understanding of OPENJSON, COALESCE, and structured updates, it’s possible to maintain the integrity of your data. The method outlined above ensures that you can modify specific attributes without losing surrounding context. Use these steps as a guide for your future JSON update tasks in SQL Server.

By implementing this systematic approach, you'll safeguard your data while achieving the necessary updates with ease.