MySQL merging json arrays in group by

I'm trying to merge a scalar array json field within a group by to have all the distinct values in one list.

Consider the following table:

CREATE TABLE transaction
(
    id                 INT UNSIGNED AUTO_INCREMENT,
    source_account_id  VARCHAR(32)                                          NOT NULL,
    target_account_ids JSON                                                 NOT NULL,
    PRIMARY KEY (id)
) ENGINE = InnoDB CHARSET utf8mb4;

source_account_ids is a simple array of strings for example '["account1", "account2"]'.
I'd like to gather all the target_account_ids of a single source to have a unified result.
For example:

And the desired result set would be:

I tried to play around with JSON_ARRAYAGG but it just adds the arrays within another array and basically results in an "endless" array.

You have to explode the array with JSON_TABLE(), then reduce the values with DISTINCT, then you can recombine them with JSON_ARRAYAGG().

select source_account_id, json_arrayagg(target_account_id) as target_account_ids
from ( 
  select distinct source_account_id, j.account_id as target_account_id
  from transaction
  cross join json_table(target_account_ids, '$[*]' columns (account_id varchar(32) path '$')) as j
) as t
group by source_account_id;

GROUP_CONCAT() supports a DISTINCT keyword in its argument, but JSON_ARRAYAGG() doesn't (this feature has been requested: https://bugs.mysql.com/bug.php?id=91993).

If this seems like a lot of needless work, or if you can't use JSON_TABLE() because you're still using MySQL 5.7, then you should store multi-valued attributes in normal rows and columns, instead of using JSON.