Understanding the Problem: Sorting JSON Array Elements in MySQL

MySQL’s json_arrayagg() function is used to aggregate arrays from multiple rows. However, it does not allow for sorting or filtering of array elements within the aggregated result set. In this blog post, we will explore how to sort and filter the elements of a JSON array using a combination of techniques such as subqueries, grouping, and string manipulation.

Background: Understanding MySQL’s json_arrayagg() Function

The json_arrayagg() function is used to aggregate arrays from multiple rows. It takes one or more tables as input, and returns an array containing the aggregated values. The basic syntax for this function is:

{< highlight mysql >}
SELECT json_arrayagg(expression)
FROM table_name;
{< /highlight >}

In the provided example, we have a query that aggregates arrays from two tables using json_arrayagg().

Understanding the Challenge: Sorting JSON Array Elements

The challenge at hand is to sort and filter the elements of the aggregated array in MySQL. This can be achieved by using a combination of techniques such as subqueries, grouping, and string manipulation.

Solution Approach

To solve this problem, we will use a three-step approach:

1. Subquery: We will first create a subquery that groups the original table’s rows based on the currency column.
2. Grouping: Next, we will group the subquery’s result by the currency column and apply a string manipulation function to sort the array elements within each group.
3. Joining: Finally, we will join the grouped result with another subquery that generates a JSON object for each row in the original table.

Step-by-Step Explanation

Let’s break down the solution step by step:

Step 1: Subquery to Group Rows

We’ll create a subquery that groups the rows based on the currency column. This subquery will use the json_arrayagg() function to aggregate arrays from two tables, just like in the original query.

with cte as (
    select json_array(datetime, rupee) AS obj, rupee as value_currency, 'rupee' as currency
    from currency
    UNION ALL 
    select json_array(datetime, pound), pound, 'pound'
    from currency
),

Step 2: Grouping and String Manipulation

Next, we’ll group the subquery’s result by the currency column and apply a string manipulation function to sort the array elements within each group. We’ll use the group_concat() function to achieve this.

cte2 as (
    select currency, 
        concat('[',
          group_concat(
            obj
            order by value_currency
          ),
          ']'
        ) obj
  from cte
  group by currency
)

Step 3: Joining Subqueries

Finally, we’ll join the grouped result with another subquery that generates a JSON object for each row in the original table. We’ll use the json_object() function to achieve this.

select json_arrayagg(obj)
from (
    select json_object(
            'currency', currency, 
            'data', CAST( obj as JSON)
       ) AS obj
  from cte2
) x

Final Query

Putting it all together, the final query that achieves the desired result is:

with cte as (
    select json_array(datetime, rupee) AS obj, rupee as value_currency, 'rupee' as currency
    from currency
    UNION ALL 
    select json_array(datetime, pound), pound, 'pound'
    from currency
),
cte2 as (
    select currency, 
        concat('[',
          group_concat(
            obj
            order by value_currency
          ),
          ']'
        ) obj
  from cte
  group by currency
)
select json_arrayagg(obj)
from (
    select json_object(
            'currency', currency, 
            'data', CAST( obj as JSON)
       ) AS obj
  from cte2
) x;

Example Use Case

The provided example demonstrates the use of this query with sample data. You can find a link to the DB Fiddle with sample data in the original problem statement.

By following these steps and using the json_arrayagg() function in combination with grouping, string manipulation, and joining subqueries, you can achieve the desired result of sorting and filtering JSON array elements in MySQL.

Last modified on 2024-10-22