SQL JSON Nested Selection

As developers, we often encounter complex data structures in our databases, and SQL queries can become cumbersome when dealing with nested JSON data. In this article, we will explore a solution to select specific fields from nested JSON without adding the parent column name.

Problem Statement

Suppose you have a database table ic_brpolicy with a column customer_data_json containing nested JSON data. You want to retrieve only certain fields from this JSON without nesting it under the parent column name.

For example, consider the following customer_data_json column:

"start_date": "2020-06-20T00:00:00Z",
"end_date": "2021-06-20T00:00:00Z",
"status": "ACTIVE",
"drivers": [{
    "driver_id": "Prop",
    "main_driver": true,
    "app_user": true
}]

You want to retrieve the start_date, end_date, and status fields without nesting them under the customer_data_json column name. Instead, you want the output to resemble this:

"start_date":"2020-06-20T00:00:00Z",
"end_date":"2021-06-20T00:00:00Z",
"status":"ACTIVE"

Solution

The solution involves using SQL Server’s JSON_QUERY function to extract specific fields from the nested JSON data. In this example, we will use a simplified table structure and values to demonstrate the concept.

First, let’s create a sample table structure and insert some sample data:

CREATE TABLE ic_brpolicy (
    start_date DATE,
    end_date DATE,
    status VARCHAR(10),
    customer_data_json NVARCHAR(MAX)
);

INSERT INTO ic_brpolicy (start_date, end_date, status, customer_data_json)
VALUES ('2020-02-27T00:00:00', '2021-02-27T00:00:00', 'ACTIVE',
        '[{"drivers": [{"driver_id": "Prop","main_driver": true,"app_user": true}]}]')
;

Next, we will use the JSON_QUERY function to extract the desired fields from the nested JSON data:

SELECT 
    p.start_date AS start_date,
    p.end_date AS end_date,
    p.status AS status,
    JSON_QUERY(p.customer_data_json, '$[0].drivers') AS drivers
FROM 
    (VALUES ('2020-02-27T00:00:00', '2021-02-27T00:00:00', 'ACTIVE',
             '[{"drivers": [{"driver_id": "Prop","main_driver": true,"app_user": true}]}]')
    ) p (start_date, end_date, status, customer_data_json)
FOR JSON PATH, WITHOUT_ARRAY_WRAPPER
;

In this query:

• We select the start_date, end_date, and status columns from the ic_brpolicy table.
• We use a subquery to retrieve the sample data.
• Inside the subquery:
  • The JSON_QUERY function is used to extract the nested JSON data from the customer_data_json column.
  • The $[0] notation specifies that we want to start extracting fields at the first level of nesting (i.e., drivers).
  • The .drivers notation extracts only the drivers array from the nested JSON data.
• The FOR JSON PATH, WITHOUT_ARRAY_WRAPPER clause is used to format the output as a flat JSON string.

Running this query will produce the desired output:

{
  "start_date":"2020-02-27T00:00:00",
  "end_date":"2021-02-27T00:00:00",
  "status":"ACTIVE",
  "drivers":
    {
      "driver_id":"Prop",
      "main_driver":true,
      "app_user":true
    }
}

Conclusion

In this article, we explored a solution to select specific fields from nested JSON data without adding the parent column name. We used SQL Server’s JSON_QUERY function to extract desired fields and format the output as a flat JSON string.

By following these steps and examples, you should be able to apply this technique to your own database queries to improve performance and reduce unnecessary data duplication.

Additional Considerations

• JSON Data Structure: The effectiveness of this solution depends on the structure of your nested JSON data. If your JSON is deeply nested or has complex relationships between fields, you may need to adjust your query accordingly.
• JSON Query Language: SQL Server’s JSON_QUERY function is part of its JSON path language. Familiarize yourself with other JSON path functions and operators to further enhance your queries.

Next Steps

To expand on this concept, consider the following:

• Handling Multiple Levels of Nesting: If you need to extract fields from multiple levels of nesting, modify your query using the $[*] notation.
• Updating Your Database Schema: Consider adding additional columns or modifying your existing schema to accommodate nested JSON data.

By exploring these advanced topics and techniques, you can further refine your SQL queries to work with complex JSON data structures.

Last modified on 2024-10-31