How to run query against JSON extract data in ClickHouse?

I have the following Clickhouse table:

id, custom, otherfield1, otherfield2.

In the custom, we put a JSON inside of it that looks like

{"activityId": 1234, "someJsonField": "blah"}

so now I want to query 10 rows where the activityId field is 1234. How do I go about doing this?

SELECT 
    custom,
    JSONExtractString(custom, 'activityId') AS ActivityId
FROM muh_table
where ActivityId = "1234"
limit 1

But then I get error, basically saying the column doesn't exist. What's the correct syntax in getting this SQL written out?

You might have to share some more details. This worked fine for me:

create or replace table zhen (
    id UInt32,
    custom String
)
ENGINE = MergeTree
PRIMARY KEY id;

INSERT INTO zhen VALUES 
    (1, '{"activityId": 1234, "someJsonField": "blah1"}'),
    (2, '{"activityId": 1234, "someJsonField": "blah2"}'),
    (3, '{"activityId": 5678, "someJsonField": "blah3"}');

SELECT 
    id,
    JSONExtractString(custom, 'activityId') AS ActivityId
FROM zhen
WHERE ActivityId = '1234';

Response is:

┌─id─┬─ActivityId─┐
│  1 │ 1234       │
│  2 │ 1234       │
└────┴────────────┘