JSON Extraction Functions
There are two extraction functions, which have their respective operators. The operators can only be used if the string is stored as the JSON logical type.
These functions supports the same two location notations as JSON Scalar functions.
| Function | Alias | Operator | Description |
|---|---|---|---|
json_exists(json, path) |
Returns true if the supplied path exists in the json, and false otherwise. |
||
json_extract(json, path) |
json_extract_path |
-> |
Extracts JSON from json at the given path. If path is a LIST, the result will be a LIST of JSON. |
json_extract_string(json, path) |
json_extract_path_text |
->> |
Extracts VARCHAR from json at the given path. If path is a LIST, the result will be a LIST of VARCHAR. |
json_value(json, path) |
Extracts JSON from json at the given path. If the json at the supplied path is not a scalar value, it will return NULL. |
Note that the arrow operator ->, which is used for JSON extracts, has a low precedence as it is also used in lambda functions.
Therefore, you need to surround the -> operator with parentheses when expressing operations such as equality comparisons (=).
For example:
SELECT ((JSON '{"field": 42}')->'field') = 42;
Warning DuckDB's JSON data type uses 0-based indexing.
Examples:
CREATE TABLE example (j JSON);
INSERT INTO example VALUES
('{ "family": "anatidae", "species": [ "duck", "goose", "swan", null ] }');
SELECT json_extract(j, '$.family') FROM example;
"anatidae"
SELECT j->'$.family' FROM example;
"anatidae"
SELECT j->'$.species[0]' FROM example;
"duck"
SELECT j->'$.species[*]' FROM example;
["duck", "goose", "swan", null]
SELECT j->>'$.species[*]' FROM example;
[duck, goose, swan, null]
SELECT j->'$.species'->0 FROM example;
"duck"
SELECT j->'species'->['0','1'] FROM example;
["duck", "goose"]
SELECT json_extract_string(j, '$.family') FROM example;
anatidae
SELECT j->>'$.family' FROM example;
anatidae
SELECT j->>'$.species[0]' FROM example;
duck
SELECT j->'species'->>0 FROM example;
duck
SELECT j->'species'->>['0','1'] FROM example;
[duck, goose]
Note that DuckDB's JSON data type uses 0-based indexing.
If multiple values need to be extracted from the same JSON, it is more efficient to extract a list of paths:
The following will cause the JSON to be parsed twice,:
Resulting in a slower query that uses more memory:
SELECT
json_extract(j, 'family') AS family,
json_extract(j, 'species') AS species
FROM example;
| family | species |
|---|---|
| "anatidae" | ["duck","goose","swan",null] |
The following produces the same result but is faster and more memory-efficient:
WITH extracted AS (
SELECT json_extract(j, ['family', 'species']) AS extracted_list
FROM example
)
SELECT
extracted_list[1] AS family,
extracted_list[2] AS species
FROM extracted;
JSON Scalar Functions
The following scalar JSON functions can be used to gain information about the stored JSON values.
With the exception of json_valid(json), all JSON functions produce an error when invalid JSON is supplied.
We support two kinds of notations to describe locations within JSON: JSON Pointer and JSONPath.
| Function | Description |
|---|---|
json_array_length(json[, path]) |
Return the number of elements in the JSON array json, or 0 if it is not a JSON array. If path is specified, return the number of elements in the JSON array at the given path. If path is a LIST, the result will be LIST of array lengths. |
json_contains(json_haystack, json_needle) |
Returns true if json_needle is contained in json_haystack. Both parameters are of JSON type, but json_needle can also be a numeric value or a string, however the string must be wrapped in double quotes. |
json_keys(json[, path]) |
Returns the keys of json as a LIST of VARCHAR, if json is a JSON object. If path is specified, return the keys of the JSON object at the given path. If path is a LIST, the result will be LIST of LIST of VARCHAR. |
json_structure(json) |
Return the structure of json. Defaults to JSON if the structure is inconsistent (e.g., incompatible types in an array). |
json_type(json[, path]) |
Return the type of the supplied json, which is one of ARRAY, BIGINT, BOOLEAN, DOUBLE, OBJECT, UBIGINT, VARCHAR, and NULL. If path is specified, return the type of the element at the given path. If path is a LIST, the result will be LIST of types. |
json_valid(json) |
Return whether json is valid JSON. |
json(json) |
Parse and minify json. |
The JSONPointer syntax separates each field with a /.
For example, to extract the first element of the array with key duck, you can do:
SELECT json_extract('{"duck": [1, 2, 3]}', '/duck/0');
1
The JSONPath syntax separates fields with a ., and accesses array elements with [i], and always starts with $. Using the same example, we can do the following:
SELECT json_extract('{"duck": [1, 2, 3]}', '$.duck[0]');
1
Note that DuckDB's JSON data type uses 0-based indexing.
JSONPath is more expressive, and can also access from the back of lists:
SELECT json_extract('{"duck": [1, 2, 3]}', '$.duck[#-1]');
3
JSONPath also allows escaping syntax tokens, using double quotes:
SELECT json_extract('{"duck.goose": [1, 2, 3]}', '$."duck.goose"[1]');
2
Examples using the anatidae biological family:
CREATE TABLE example (j JSON);
INSERT INTO example VALUES
('{ "family": "anatidae", "species": [ "duck", "goose", "swan", null ] }');
SELECT json(j) FROM example;
{"family":"anatidae","species":["duck","goose","swan",null]}
SELECT j.family FROM example;
"anatidae"
SELECT j.species[0] FROM example;
"duck"
SELECT json_valid(j) FROM example;
true
SELECT json_valid('{');
false
SELECT json_array_length('["duck", "goose", "swan", null]');
4
SELECT json_array_length(j, 'species') FROM example;
4
SELECT json_array_length(j, '/species') FROM example;
4
SELECT json_array_length(j, '$.species') FROM example;
4
SELECT json_array_length(j, ['$.species']) FROM example;
[4]
SELECT json_type(j) FROM example;
OBJECT
SELECT json_keys(j) FROM example;
[family, species]
SELECT json_structure(j) FROM example;
{"family":"VARCHAR","species":["VARCHAR"]}
SELECT json_structure('["duck", {"family": "anatidae"}]');
["JSON"]
SELECT json_contains('{"key": "value"}', '"value"');
true
SELECT json_contains('{"key": 1}', '1');
true
SELECT json_contains('{"top_key": {"key": "value"}}', '{"key": "value"}');
true
JSON Aggregate Functions
There are three JSON aggregate functions.
| Function | Description |
|---|---|
json_group_array(any) |
Return a JSON array with all values of any in the aggregation. |
json_group_object(key, value) |
Return a JSON object with all key, value pairs in the aggregation. |
json_group_structure(json) |
Return the combined json_structure of all json in the aggregation. |
Examples:
CREATE TABLE example1 (k VARCHAR, v INTEGER);
INSERT INTO example1 VALUES ('duck', 42), ('goose', 7);
SELECT json_group_array(v) FROM example1;
[42, 7]
SELECT json_group_object(k, v) FROM example1;
{"duck":42,"goose":7}
CREATE TABLE example2 (j JSON);
INSERT INTO example2 VALUES
('{"family": "anatidae", "species": ["duck", "goose"], "coolness": 42.42}'),
('{"family": "canidae", "species": ["labrador", "bulldog"], "hair": true}');
SELECT json_group_structure(j) FROM example2;
{"family":"VARCHAR","species":["VARCHAR"],"coolness":"DOUBLE","hair":"BOOLEAN"}
Transforming JSON to Nested Types
In many cases, it is inefficient to extract values from JSON one-by-one.
Instead, we can “extract” all values at once, transforming JSON to the nested types LIST and STRUCT.
| Function | Description |
|---|---|
json_transform(json, structure) |
Transform json according to the specified structure. |
from_json(json, structure) |
Alias for json_transform. |
json_transform_strict(json, structure) |
Same as json_transform, but throws an error when type casting fails. |
from_json_strict(json, structure) |
Alias for json_transform_strict. |
The structure argument is JSON of the same form as returned by json_structure.
The structure argument can be modified to transform the JSON into the desired structure and types.
It is possible to extract fewer key/value pairs than are present in the JSON, and it is also possible to extract more: missing keys become NULL.
Examples:
CREATE TABLE example (j JSON);
INSERT INTO example VALUES
('{"family": "anatidae", "species": ["duck", "goose"], "coolness": 42.42}'),
('{"family": "canidae", "species": ["labrador", "bulldog"], "hair": true}');
SELECT json_transform(j, '{"family": "VARCHAR", "coolness": "DOUBLE"}') FROM example;
{'family': anatidae, 'coolness': 42.420000}
{'family': canidae, 'coolness': NULL}
SELECT json_transform(j, '{"family": "TINYINT", "coolness": "DECIMAL(4, 2)"}') FROM example;
{'family': NULL, 'coolness': 42.42}
{'family': NULL, 'coolness': NULL}
SELECT json_transform_strict(j, '{"family": "TINYINT", "coolness": "DOUBLE"}') FROM example;
Invalid Input Error: Failed to cast value: "anatidae"