Working with Multiple Keys in JSON and Returning Only Rows with Values in PostgreSQL 9.5

As a technical blogger, I’ve come across many queries where dealing with JSON data has proven challenging. In this article, we’ll explore how to find multiple keys in multiple JSON rows and return only those rows that have some value for specific keys.

Introduction

JSON (JavaScript Object Notation) is a popular data interchange format used extensively in modern applications. PostgreSQL, being one of the leading relational databases, supports storing JSON data. However, when working with JSON, it can be daunting to extract specific values or keys from these complex data structures.

Background and Context

The provided Stack Overflow question illustrates a common problem faced by many developers who work with JSON data in PostgreSQL. The question presents two cases:

Case 1: A single key-value pair needs to be extracted, and the result should only contain rows where that specific key has a non-null value.
Case 2: Multiple keys need to be extracted, and the result should include rows where any of those keys have a non-null value.

In both cases, the desired outcome is to return only those rows from the original data set where at least one of the specified keys has a non-null value.

Solution Overview

To solve these problems, we can utilize PostgreSQL’s advanced JSON processing capabilities and aggregation functions. Here are the steps for each case:

Case 1: Single Key with Non-Null Value

For the first case, you can use a simple WHERE clause with an inequality operator (<>) to exclude rows where the specified key has a null value.

SELECT 
    application_id,
    processing_json -> 'official_form_attributes' - '>' '81488'
FROM 
    schm_ka.processing_data_json
WHERE 
    application_id = 9356416
    AND processing_json -> 'official_form_attributes' <> '';

However, if you want to ensure that the value for the specified key is not null and also exclude rows with a null value for any other keys in the JSON object, you’ll need to use a more complex query.

Case 2: Multiple Keys with Non-Null Values

For the second case, where multiple keys are involved, you can use PostgreSQL’s array and aggregation functions to achieve the desired outcome. Here’s an example:

SELECT 
    application_id,
    max(processing_json -> 'official_form_attributes' - '>' '81488') as key_value_1,
    max(processing_json -> 'official_form_attributes' - '>' '81315') as key_value_2
FROM 
    schm_ka.processing_data_json
WHERE 
    application_id = 9356416
    AND processing_json -> 'official_form_attributes' ?| array['81488', '81315']
GROUP BY 
    application_id;

In this query, the ?| operator tests if any of the keys in the provided array is present in the JSON value. The aggregation function (max) then collapses the rows into one row for each group, effectively returning only those rows where at least one of the specified keys has a non-null value.

Conclusion

When working with JSON data in PostgreSQL, being able to extract specific values or keys efficiently can significantly impact your queries’ performance and readability. By using advanced features like inequality operators, aggregation functions, and array operators, you can effectively solve complex problems involving multiple keys in JSON data.

Tips for Working with JSON Data:

Always use the correct operator (->, ?|, etc.) when working with JSON values.
Use aggregation functions (e.g., max, min) to collapse rows into a single row, especially when dealing with arrays or complex keys.
Consider using the JSONB data type instead of JSON for improved performance and compatibility with indexing.

By mastering these techniques and leveraging PostgreSQL’s advanced JSON capabilities, you can simplify your queries, improve performance, and unlock the full potential of your relational database.

Last modified on 2023-06-29