til/mysql/set-value-on-null-json-column.md

Set Value On Null JSON Column

To set a key-value pair on a JSON field, you can reach for MySQL's json_set or one of the other JSON setter functions.

However, if the JSON field you are updating is null, you might get an unexpected result.

> update User
    set metadata = json_set(metadata, '$.discord_id', 'discord_123')
    where id = 123;

Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

We can see that the where clause matched on a single row as expected, but right above that it says 0 rows affected.

What happened?

The json_set function is not able to set a key-value pair on null. It needs a JSON object to work on.

There are a number of ways to get around this. I find that coalesce is a natural way to handle this. If metadata happens to be null, then we coalesce it to '{}' (an empty object).

> update User
    set metadata = json_set(coalesce(metadata, '{}'), '$.discord_id', 'discord_123')
    where id = 123;

Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

It updates as expected. That same statement will work on a row where metadata already contains a JSON object since the coalesce will resolve to that instead of the empty object.