MongoDB {"$exists": false} equivalent in Postgres

assuming you have a table (note json type):

create table MY_TABLE (
    ID int not null,
    JSON_VALUE json not null 
);

with data:

insert into MY_TABLE (ID, JSON_VALUE) values (1, '{"myKey":"should be found", "otherKey":"123"}');
insert into MY_TABLE (ID, JSON_VALUE) values (2, '{"somethingElse":"should not"}');

you can use postgre json function to get all the IDs of

select ID from 
(select ID, json_extract_path(JSON_VALUE, 'myKey') AS keys from MY_TABLE) t

where keys is null -- equivalent to {'myKey': { '$exists': False }}
-- or
where keys is not null -- equivalent to {'myKey': { '$exists': True }}

to clarify the result of sub-query: select ID, json_extract_path(JSON_VALUE, 'myKey') AS keys from MY_TABLE would be:

id  keys
1   should be found
2   (null)

If you want to check a top level field you can add a where clause:

select * from myDocs
where column_name is null
limit 1;

And if you want to check a subfield of a json field you can use the jsonb ? operator:

select * from myDocs
where column_name::jsonb ? 'myKey' is false -- or
-- where column_name::jsonb ? 'myKey' = false
limit 1;

jsonb ? text → boolean

Does the text string exist as a top-level key or array element within the JSON value?

Example of table

CREATE TABLE example_table(
    id INT NOT NULL,
    json_data JSON NOT NULL
);

Query:

SELECT * FROM (SELECT id, json_extract_path(json_data, 'myKey') AS jsonKey
FROM example_table) t WHERE jsonKey IS NULL

similar to:

db.myDocs.find({'myKey': { '$exists': false }})

you can view more information on the commands for Postgres JSON