Summary: in this tutorial, you will learn how to use the PostgreSQL row_to_json() function to convert an SQL composite value to a JSON object.

Introduction to the PostgreSQL row_to_json() function

The row_to_json() function allows you to convert an SQL composite value into a JSON object.

Here’s the syntax of the row_to_json() function:

row_to_json ( record [, boolean ] ) → json

In this syntax:

  • record is an SQL composite value that you want to convert into a JSON object.
  • boolean if true, the function will add a line feed between top-level elements.

The row_to_json() function will return a JSON object.

PostgreSQL row_to_json() function examples

Let’s take some examples of using the row_to_json() function.

1) Basic row_to_json() function example

The following example uses the row_to_json() function to convert a row into a JSON object:

SELECT row_to_json(row('John',20));

Output:

row_to_json
-----------------------
 {"f1":"John","f2":20}
(1 row)

In this example, we use the row() function to create a composite value made up of multiple columns.

The row_to_json() function returns an object whose keys are automatically generated f1 and f2 with the values from the composite values.

2) Using the row_to_json() function with table data

We’ll use the film table from the sample database:

The following example uses the row_to_json() function to convert the title and length of each film in the film table into a JSON object:

SELECT
  row_to_json(t) film
FROM
  (
    SELECT
      title,
      length
    FROM
      film
    ORDER BY
      title
  ) t;

Output:

film
------------------------------------------------------
 {"title":"Academy Dinosaur","length":86}
 {"title":"Ace Goldfinger","length":48}
 {"title":"Adaptation Holes","length":50}
 {"title":"Affair Prejudice","length":117}
 {"title":"African Egg","length":130}
 {"title":"Agent Truman","length":169}
...

How it works.

  • The subquery retrieves the title and length from the film table.
  • The outer query uses the row_to_json() to convert each row returned by the subquery into a JSON object.

Note that you can use a common table expression (CTE) instead of a subquery to achieve the same result:

WITH film_cte AS (
  SELECT
    title,
    length
  FROM
    film
  ORDER BY
    title
)
SELECT
  row_to_json(film_cte)
FROM
  film_cte;

Summary

  • Use the row_to_json() function to convert an SQL composite value to a JSON object.