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# Postgres json_object() function

Creates a JSON object from key-value pairs

The `json_object` function in Postgres is used to create a `JSON` object from a set of key-value pairs. It is particularly useful when you need to generate `JSON` data dynamically from existing table data or input parameters.

## Function signature

```sql
json_object(keys TEXT[], values TEXT[]) -> JSON
-- or --
json_object(keys_values TEXT[]) -> JSON
```

This function takes two text arrays as input: one for keys and one for values. Both arrays must have the same number of elements, as each key is paired with the corresponding value to construct the `JSON` object.

Alternatively, you can pass a single text array containing both keys and values. In this case, alternate elements in the array are treated as keys and values, respectively.

## Example usage

Consider a scenario where you run a library and have a table that tracks details for each book.

The table with some sample data can be set up as shown:

```sql
-- Test database table for a bookstore inventory
CREATE TABLE book_inventory (
    book_id INT,
    title TEXT,
    author TEXT,
    price NUMERIC,
    genre TEXT
);

-- Inserting some test data into `book_inventory`
INSERT INTO book_inventory VALUES
(101, 'The Great Gatsby', 'F. Scott Fitzgerald', 18.99, 'Classic'),
(102, 'Invisible Man', 'Ralph Ellison', 15.99, 'Novel');
```

When querying this dataset, the frontend client might want to present the data in a different way. Say you want the catalog information just as the list of book names while combining the rest of the fields into a single `metadata` attribute. You can do so as shown here:

```sql
SELECT book_id, title, json_object(
  ARRAY['author', 'genre'],
  ARRAY[author, genre]
) AS metadata
FROM book_inventory;
```

This query returns the following result:

```text
| book_id | title            | metadata                                   |
|---------|------------------|--------------------------------------------|
| 101     | The Great Gatsby | {"author" : "F. Scott Fitzgerald",         |
|         |                  |  "genre" : "Classic"}                      |
| 102     | Invisible Man    | {"author" : "Ralph Ellison",               |
|         |                  |  "genre" : "Novel"}                        |
```

## Advanced examples

### Creating nested JSON objects with `json_object`

You could use `json_object` to create nested `JSON` objects for representing more complex data. However, since `json_object` only expects text values for each key, we will need to combine it with other `JSON` functions like `json_build_object`. For example:

```sql
SELECT json_build_object(
  'title', title,
  'author', json_object(ARRAY['name', 'genre'], ARRAY[author, genre])
) AS book_info
FROM book_inventory;
```

This query returns the following result:

```text
| book_info                                                                                        |
|--------------------------------------------------------------------------------------------------|
| {"title" : "The Great Gatsby", "author" : {"name" : "F. Scott Fitzgerald", "genre" : "Classic"}} |
| {"title" : "Invisible Man", "author" : {"name" : "Ralph Ellison", "genre" : "Novel"}}            |
```

## Additional considerations

### Gotchas and footguns

- Ensure both keys and values arrays have the same number of elements. Mismatched arrays will result in an error. Or, if passing in a single key-value array, ensure that the array has an even number of elements.
- Be aware of data type conversions. Since `json_object` expects text arrays, you may need to explicitly cast non-text data types to text.

### Alternative functions

- [jsonb_object](https://www.postgresql.org/docs/current/functions-json.html) - Same functionality as `json_object`, but returns a `JSONB` object instead of `JSON`.
- [row_to_json](https://www.postgresql.org/docs/current/functions-json.html) - It can be used to create a `JSON` object from a table row (or a row of a composite type) without needing to specify keys and values explicitly. Although, it is less flexible than `json_object` since all fields in the row are included in the `JSON` object.
- [json_build_object](https://neon.com/docs/functions/json_build_object) - Similar to `json_object`, but allows for more flexibility in constructing the `JSON` object, as it can take a variable number of arguments in the form of key-value pairs.
- [json_object_agg](https://www.postgresql.org/docs/current/functions-json.html) - It is used to aggregate the key-value pairs from multiple rows into a single `JSON` object. In contrast, `json_object` outputs a `JSON` object for each row.

## Resources

- [PostgreSQL documentation: JSON functions](https://www.postgresql.org/docs/current/functions-json.html)

---

## Related docs (JSON functions)

- [array_to_json](https://neon.com/docs/functions/array_to_json)
- [json](https://neon.com/docs/functions/json)
- [json_agg](https://neon.com/docs/functions/json_agg)
- [json_array_elements](https://neon.com/docs/functions/json_array_elements)
- [json_build_object](https://neon.com/docs/functions/json_build_object)
- [json_each](https://neon.com/docs/functions/json_each)
- [json_exists](https://neon.com/docs/functions/json_exists)
- [json_extract_path](https://neon.com/docs/functions/json_extract_path)
- [json_extract_path_text](https://neon.com/docs/functions/json_extract_path_text)
- [json_populate_record](https://neon.com/docs/functions/json_populate_record)
- [json_query](https://neon.com/docs/functions/json_query)
- [json_scalar](https://neon.com/docs/functions/json_scalar)
- [json_serialize](https://neon.com/docs/functions/json_serialize)
- [json_table](https://neon.com/docs/functions/json_table)
- [json_to_record](https://neon.com/docs/functions/json_to_record)
- [json_value](https://neon.com/docs/functions/json_value)
- [jsonb_array_elements](https://neon.com/docs/functions/jsonb_array_elements)
- [jsonb_each](https://neon.com/docs/functions/jsonb_each)
- [jsonb_extract_path](https://neon.com/docs/functions/jsonb_extract_path)
- [jsonb_extract_path_text](https://neon.com/docs/functions/jsonb_extract_path_text)
- [jsonb_object](https://neon.com/docs/functions/jsonb_object)
- [jsonb_populate_record](https://neon.com/docs/functions/jsonb_populate_record)
- [jsonb_to_record](https://neon.com/docs/functions/jsonb_to_record)