Druid supports nested columns, which provide optimized storage and indexes for nested data structures. See [Nested columns](./nested-columns.md) for more information.
You can use the following JSON functions to extract, transform, and create `COMPLEX<json>` values.
|`JSON_KEYS(expr, path)`| Returns an array of field names from `expr` at the specified `path`.|
|`JSON_OBJECT(KEY expr1 VALUE expr2[, KEY expr3 VALUE expr4, ...])` | Constructs a new `COMPLEX<json>` object. The `KEY` expressions must evaluate to string types. The `VALUE` expressions can be composed of any input type, including other `COMPLEX<json>` values. `JSON_OBJECT` can accept colon-separated key-value pairs. The following syntax is equivalent: `JSON_OBJECT(expr1:expr2[, expr3:expr4, ...])`.|
|`JSON_PATHS(expr)`| Returns an array of all paths which refer to literal values in `expr` in JSONPath format. |
|`JSON_QUERY(expr, path)`| Extracts a `COMPLEX<json>` value from `expr`, at the specified `path`. |
|`JSON_VALUE(expr, path [RETURNING sqlType])`| Extracts a literal value from `expr` at the specified `path`. If you specify `RETURNING` and an SQL type name (such as `VARCHAR`, `BIGINT`, `DOUBLE`, etc) the function plans the query using the suggested type. Otherwise, it attempts to infer the type based on the context. If it can't infer the type, it defaults to `VARCHAR`.|
|`PARSE_JSON(expr)`|Parses `expr` into a `COMPLEX<json>` object. This operator deserializes JSON values when processing them, translating stringified JSON into a nested structure. If the input is not a `VARCHAR` or it is invalid JSON, this function will result in an error.|
|`TRY_PARSE_JSON(expr)`|Parses `expr` into a `COMPLEX<json>` object. This operator deserializes JSON values when processing them, translating stringified JSON into a nested structure. If the input is not a `VARCHAR` or it is invalid JSON, this function will result in a `NULL` value.|
|`TO_JSON_STRING(expr)`|Serializes `expr` into a JSON string.|
### JSONPath syntax
Druid supports a subset of the [JSONPath syntax](https://github.com/json-path/JsonPath/blob/master/README.md) operators, primarily limited to extracting individual values from nested data structures.
|Operator|Description|
| --- | --- |
|`$`| Root element. All JSONPath expressions start with this operator. |
|`.<name>`| Child element in dot notation. |
|`['<name>']`| Child element in bracket notation. |
|`[<number>]`| Array index. |
Consider the following example input JSON:
```json
{"x":1, "y":[1, 2, 3]}
```
- To return the entire JSON object:<br>
`$` -> `{"x":1, "y":[1, 2, 3]}`
- To return the value of the key "x":<br>
`$.x` -> `1`
- For a key that contains an array, to return the entire array:<br>
`$['y']` -> `[1, 2, 3]`
- For a key that contains an array, to return an item in the array:<br>
