Efficiently Combine Multiple JSON Columns into a JSON Array in MariaDB

• You want to retrieve these JSON objects and combine them into a single JSON array as the query result.
• You have a MariaDB table with one or more columns containing JSON data.

Solution Using JSON_ARRAYAGG:

MariaDB provides the JSON_ARRAYAGG function, introduced in version 10.5.0, to achieve this. Here's the basic structure:

SELECT JSON_ARRAYAGG(column_or_expression) AS json_array
FROM your_table;

• AS json_array: Aliases the resulting JSON array for clarity.
• JSON_ARRAYAGG: Aggregates the JSON values from each row into a single JSON array.
• column_or_expression: This can be a single JSON column name or an expression that evaluates to a JSON value.

Example:

Assuming you have a table data with columns id and json_data:

CREATE TABLE data (
  id INT PRIMARY KEY,
  json_data JSON
);

INSERT INTO data (id, json_data) VALUES
  (1, '{"name": "Alice", "age": 30}'),
  (2, '{"name": "Bob", "city": "New York"}'),
  (3, '{"name": "Charlie", "occupation": "Developer"}');

SELECT JSON_ARRAYAGG(json_data) AS json_array
FROM data;

This query will output:

[{"name": "Alice", "age": 30}, {"name": "Bob", "city": "New York"}, {"name": "Charlie", "occupation": "Developer"}]

Explanation:

1. JSON_ARRAYAGG iterates through each row in the data table.
2. For each row, it extracts the value from the json_data column.
3. These individual JSON objects are added to the growing array.
4. Finally, the complete JSON array is returned as the query result.

Additional Considerations:

• Ordering the Array: You can use the ORDER BY clause within JSON_ARRAYAGG to sort the resulting array based on specific criteria within the JSON objects.
• Handling NULL Values: The JSON_ARRAYAGG function ignores NULL values by default. If you want to include them in the array as null, use JSON_ARRAYAGG(COALESCE(column_or_expression, 'null')).

CREATE TABLE products (
  id INT PRIMARY KEY,
  details JSON
);

INSERT INTO products (id, details) VALUES
  (1, '{"name": "Shirt", "price": 25.99}'),
  (2, '{"name": "Hat", "price": 19.50}'),
  (3, NULL);  -- Include a NULL value

SELECT JSON_ARRAYAGG(details) AS product_data
FROM products;

This code retrieves the details (JSON) column from the products table and combines them into a JSON array named product_data. It will output:

[{"name": "Shirt", "price": 25.99}, {"name": "Hat", "price": 19.50}, null]

Handling NULL Values (Including NULLs in the Array):

SELECT JSON_ARRAYAGG(COALESCE(details, 'null')) AS product_data
FROM products;

This code modifies the previous example by using COALESCE(details, 'null') within JSON_ARRAYAGG. This ensures that even NULL values in the details column are included in the array as null.

The output will be:

[{"name": "Shirt", "price": 25.99}, {"name": "Hat", "price": 19.50}, null]

Ordering the Array by Price (Descending):

SELECT JSON_ARRAYAGG(details ORDER BY JSON_EXTRACT(details, '$.price') DESC) AS product_data
FROM products;

This code sorts the JSON objects within the array based on the price value extracted using JSON_EXTRACT. The ORDER BY clause specifies descending order (highest price first).

[{"name": "Shirt", "price": 25.99}, {"name": "Hat", "price": 19.50}, null]  -- Order is preserved

• Functionality: This method works by concatenating individual JSON objects into a single string using CONCAT and then wrapping it in square brackets [] to form a JSON array. However, it has limitations:
  • Doesn't handle nested quotes or special characters within the JSON data itself (requires escaping).
  • Not as robust or secure as dedicated JSON functions.

SELECT CONCAT('[', GROUP_CONCAT(json_data SEPARATOR ','), ']') AS json_array
FROM your_table;

User-Defined Functions (UDFs) for Complex Logic:

• Functionality: If you require more complex manipulation or transformations on the JSON data before aggregation, you can create custom UDFs using functions like JSON_EXTRACT and JSON_SET.

Example (Illustrative - Specific implementation depends on your logic):

CREATE FUNCTION combine_json(json_data1 JSON, json_data2 JSON) RETURNS JSON
BEGIN
  DECLARE combined_json JSON;

  SET combined_json = JSON_MERGE(json_data1, json_data2);  -- Example logic

  RETURN combined_json;
END;

SELECT JSON_ARRAYAGG(combine_json(json_data1, json_data2)) AS json_array
FROM your_table;

Temporary Tables for Complex Processing:

• Functionality: For very complex processing involving multiple joins and aggregations, you can create a temporary table to store intermediate results and then construct the final JSON array using JSON_ARRAYAGG.

Example (Structure depends on your specific processing):

CREATE TEMPORARY TABLE temp_data (
  id INT,
  combined_json JSON
);

-- Insert logic to populate temp_data with processed JSON

SELECT JSON_ARRAYAGG(combined_json) AS json_array
FROM temp_data;

Choosing the Right Method:

• Employ temporary tables for very complex processing flows involving multiple joins and intermediate results.
• Use UDFs when you need to perform specific transformations on the JSON data before aggregation.
• Consider CONCAT and GROUP_CONCAT for very basic scenarios, but be cautious of limitations related to data integrity.
• JSON_ARRAYAGG is generally the preferred method for its simplicity, efficiency, and native JSON handling capabilities (available in MariaDB 10.5.0 and later).