MySQL 8.0

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select version();


version()


8.0.36


create table races (id serial, data json);
insert into races (data) values ('{
"age": "4",
"course": "Ascot",
"horses": [
    {
        "number": "6"
    },
    {
        "number": "5"
    },
    {
        "number": "2", "foo":"bar"
    },
    {
        "number": "7"
    },
    {
        "number": "9"
    }
],
"id_race": "242723",
"canceled": "0",
"distance": "5f",
"finished": "0",
"finish_time": ""

}')
select id from races where json_contains(data, '{"number":"2"}', '$.horses')


id


1


select id from races where json_contains(data, '{"number":"3"}', '$.horses')


id