Summary: The json_scalar() function, introduced in PostgreSQL 17, converts a single SQL scalar value (integer, float, boolean, text, timestamp, or NULL) into its corresponding JSON scalar type with correct formatting. Use it when building JSON output with json_build_object() or json_build_array() and you need explicit, type-safe conversion rather than implicit casting. Numbers map to JSON numbers, booleans to JSON booleans, timestamps to ISO 8601 strings with timezone, and NULL to SQL NULL.

Convert SQL Scalar Values to JSON Scalar Values

The json_scalar() function introduced in PostgreSQL 17 provides a straightforward way to convert SQL scalar values into their JSON equivalents. Use it when you need to ensure proper type conversion and formatting of individual values for JSON output.

Use json_scalar() when you need to:

• Convert SQL numbers to JSON numbers
• Format timestamps as JSON strings
• Convert SQL booleans to JSON booleans
• Ensure proper null handling in JSON context

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The json_scalar() function uses the following syntax:

json_scalar(expression) → json

Parameters:

• expression: Any SQL scalar value to be converted to a JSON scalar value

Let's explore various ways to use the json_scalar() function with different types of input values.

-- Convert integer
SELECT json_scalar(42);

# | json_scalar
---------------
1 | 42

-- Convert floating-point number
SELECT json_scalar(123.45);

# | json_scalar
---------------
1 | 123.45

-- Convert text
SELECT json_scalar('Hello, World!');

# |     json_scalar
--------------------
1 | "Hello, World!"

-- Convert timestamp
SELECT json_scalar(CURRENT_TIMESTAMP);

# |            json_scalar
---------------------------------------
1 | "2024-12-04T06:19:14.458444+00:00"

-- Convert date
SELECT json_scalar(CURRENT_DATE);

# |  json_scalar
----------------
1 | "2024-12-04"

-- Convert boolean true
SELECT json_scalar(true);

# | json_scalar
--------------
1 | true

-- Convert NULL value
SELECT json_scalar(NULL);

# | json_scalar
--------------
1 |

-- Create a JSON object with properly formatted values
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name TEXT,
    created_at TIMESTAMP WITH TIME ZONE
);

INSERT INTO users (name, created_at)
VALUES
    ('Alice', '2024-12-04T14:30:45.000000+00:00'),
    ('Bob', '2024-12-04T15:30:45.000000+00:00');

SELECT json_build_object(
    'id', json_scalar(id),
    'name', json_scalar(name),
    'created_at', json_scalar(created_at)
)
FROM users;

# |                              json_build_object
-----------------------------------------------------------------------------------
1 | {"id" : 3, "name" : "Alice", "created_at" : "2024-12-04T14:30:45.000000+00:00"}
2 | {"id" : 4, "name" : "Bob", "created_at" : "2024-12-04T15:30:45.000000+00:00"}

-- Convert mixed data types in a single query
SELECT json_build_array(
    json_scalar(42),
    json_scalar('text'),
    json_scalar(CURRENT_TIMESTAMP),
    json_scalar(NULL)
);

# |                 json_build_array
----------------------------------------------------------
1 | [42, "text", "2024-12-04T06:25:29.928376+00:00", null]

The function follows these conversion rules:

1. NULL -> SQL NULL
2. Numbers → JSON numbers (preserving exact value)
3. Booleans → JSON booleans
4. All other types → JSON strings with appropriate formatting:
   • Timestamps include timezone when available
   • Text is properly escaped according to JSON standards

• json_build_object() function documentation
• PostgreSQL JSON functions documentation
• PostgreSQL data type formatting

• array_to_json
• json
• json_agg
• json_array_elements
• json_build_object
• json_each
• json_exists
• json_extract_path
• json_extract_path_text
• json_object
• json_populate_record
• json_query
• json_serialize
• json_table
• json_to_record
• json_value
• jsonb_array_elements
• jsonb_each
• jsonb_extract_path
• jsonb_extract_path_text
• jsonb_object
• jsonb_populate_record
• jsonb_to_record

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