How to query a json column for empty objects?

In databases, particularly in those that support JSON types like PostgreSQL, querying empty objects in JSON columns is a common requirement. Here are several methods to identify empty objects stored in JSON columns {}.

Method 1: Using Native JSON Operations and Functions

We can use database-provided JSON-related functions to query empty objects. For example, in PostgreSQL, assume we have a table named data with a JSON column named attributes. We can use the following SQL query to find all records where the attributes column contains an empty object:

sql
SELECT * FROM data
WHERE attributes = '{}';

This method is straightforward, leveraging PostgreSQL's internal capabilities for JSON data.

Method 2: Using JSON Processing Functions

Some databases support specific functions for handling JSON data, such as the jsonb_object_keys function (in PostgreSQL versions that support jsonb). This function returns all keys in a JSON object. If a JSON object is empty, the jsonb_object_keys function returns an empty array. Based on this property, we can write the following query:

sql
SELECT * FROM data
WHERE jsonb_object_keys(attributes) = '{}';

Or for cases with no keys:

sql
SELECT * FROM data
WHERE cardinality(array(SELECT jsonb_object_keys(attributes))) = 0;

This method involves extracting the keys of the JSON object and counting them; if the count is zero, it indicates an empty object.

Method 3: Using JSON Length Functions

If your database supports checking the length of JSON objects (such as MySQL's JSON_LENGTH function), you can use a similar approach to query empty JSON objects:

sql
SELECT * FROM data
WHERE JSON_LENGTH(attributes) = 0;

Summary

The choice of method depends on the database you are using and its support for JSON data types. In practical applications, it is generally recommended to leverage the database's native JSON capabilities, as this is typically more efficient and aligns better with database design.

Ensure that you understand and test your database's specific support for JSON before applying these queries, as different database systems may have varying support and functions for JSON.