How can I get the type of a JSON value in Presto? - sql

Question

I have a JSON string that I am querying with Presto and I want to aggregate by the types of values. To do this I need to get the value type. Specifically, for JSON like:
{
"a": 1,
"b": "a",
"c": true,
"d": [ 1 ],
"e": { "f": "g" },
}
I would like to get the value at $.a is an integer, the value at $.b is a string, etc. (The information doesn't need to be nested so it would be good enough to know that $.d is an array and $.e is an object).
typeof appears to return only varchar or json, depending on how you extract JSON from a string:
SELECT
typeof(json_extract(j, '$.a')),
typeof(json_extract_scalar(j, '$.a'))
FROM (SELECT '{"a":1,"b":"a","c":true,"d":[1]}' AS j);
gives me:
_col0 _col1
json varchar(32)
How can I determine the JSON value type for one of these fields?

Answer 1

json_extract gets a value in a JSON string as the json type.
json_format takes a json value and converts it back to a JSON string.
I think you can hack these two things together to get the type of a value at a position by examining the formatted string.
CASE
WHEN substr(value, 1, 1) = '{' THEN 'object'
WHEN substr(value, 1, 1) = '[' THEN 'array'
WHEN substr(value, 1, 1) = '"' THEN 'string'
WHEN substr(value, 1, 1) = 'n' THEN 'null'
WHEN substr(value, 1, 1) = 't' THEN 'boolean'
WHEN substr(value, 1, 1) = 'f' THEN 'boolean'
ELSE 'number'
END AS t
(where value is json_format(json_extract(json_string, '$.your_json_path')))
Here's an example:
SELECT obj, value,
CASE
WHEN substr(value, 1, 1) = '{' THEN 'object'
WHEN substr(value, 1, 1) = '[' THEN 'array'
WHEN substr(value, 1, 1) = '"' THEN 'string'
WHEN substr(value, 1, 1) = 'n' THEN 'null'
WHEN substr(value, 1, 1) = 't' THEN 'boolean'
WHEN substr(value, 1, 1) = 'f' THEN 'boolean'
ELSE 'number'
END AS t
FROM (
SELECT obj, json_format(json_extract(obj, '$.a')) AS value
FROM (
VALUES
('{"a":1}'),
('{"a":2.5}'),
('{"a":"abc"}'),
('{"a":true}'),
('{"a":false}'),
('{"a":null}'),
('{"a":[1]}'),
('{"a":{"h":"w"}}')
) AS t (obj)
)
Which produces this result:
obj value t
{"a":1} 1 number
{"a":2.5} 2.5 number
{"a":"abc"} "abc" string
{"a":true} true boolean
{"a":false} false boolean
{"a":null} null null
{"a":[1]} [1] array
{"a":{"h":"w"}} {"h":"w"} object