Is it possible to remove a JSON array element by value in MySQL?

In MySQL, it's possible to remove an array element by index:
SELECT JSON_REMOVE('["a", "b", "c"]', '$[1]') result;
+------------+
| result |
+------------+
| ["a", "c"] |
+------------+


Is it possible to remove by value instead - in this case the element(s) matching the string "b"?

Based on the documentation, it's possible to find the index via JSON_SEARCH and then passing it to JSON_REMOVE, however, I was wondering if there's an expression/function to do it in a single function.

As of writing this, MySQL has no dedicated JSON function to allow you to remove a JSON array element by its value. You have to use whatever functions are available to come up with a custom expression to achieve your goal. Depending on the requirements, you might even want to create a custom function.

More specifically, if the requirement is to remove the first occurrence of a value, then one way to do it is like this:

SET @value := 'b';

SELECT
  j.j AS before_remove
, JSON_REMOVE(
    j.j
  , JSON_UNQUOTE(
      JSON_SEARCH(
        j.j
      , 'one'
      , @value
      )
    )
  ) AS after_remove
FROM
  (SELECT '["a", "b", "c", "b"]') AS j (j)
;

Returns:

before_remove
after_remove

["a", "b", "c", "b"]
["a", "c", "b"]

JSON_SEARCH returns a quoted path to the value found ("$[1]" in this case), JSON_UNQUOTE removes the quotes (yielding $[1]), following which the path can be fed to JSON_REMOVE.

If you want to remove an arbitrary occurrence of a value, the expression becomes somewhat more complex:

SET @value := 'b';
SET @occurrence := 2;

SELECT
  j.j AS before_remove
, JSON_REMOVE(
    j.j
  , JSON_UNQUOTE(
      JSON_EXTRACT(
        JSON_SEARCH(
          j.j
        , 'all'
        , @value
        )
      , CONCAT('$[', @occurrence - 1, ']')
      )
    )
  ) AS after_remove
FROM
  (SELECT '["a", "b", "c", "b"]') AS j (j)
;

Output:

before_remove
after_remove

["a", "b", "c", "b"]
["a", "b", "c"]

In this case, depending on the number of entries found, JSON_SEARCH may return a JSON array of paths rather than a single path. Before applying JSON_UNQUOTE, therefore, we need to extract the specific path from the array matching the required occurrence, for which we can use JSON_EXTRACT. The CONCAT expression returns the corresponding path for JSON_EXTRACT (and then we get a path to use with JSON_REMOVE – yes, I know, it is a little confusing).

Finally, in order to be able to remove all occurrences, you need to loop over the results of the path array returned by JSON_SEARCH and either sequentially run JSON_REMOVE with each path or somehow build and evaluate a single dynamic JSON_REMOVE expression including all paths. Either way you need to do this in a custom-created function.

Alternatively you could try solving this in a set-based manner, for instance using the JSON_TABLE function to turn the path array – output of JSON_SEARCH – into a row set, one path per row, and then using a recursive CTE on the set, applying each path one by one and getting the last result as the final JSON.

The options are there and I am leaving the solution as an exercise for the reader. I will only add that both queries above are available online at db<>fiddle.

StackExchange Database Administrators Q#293864, answer score: 4