Question

I have two tables drivers and drivers_names. What I want is for every driver I select from the first table to have a random name from the second, but what I get is one name for all drivers in the result. Yes, it is different every time but is one for all. Here is my query, I’m using postgresql.

SELECT
    drivers.driver_id AS drivers_driver_id,
    (
        SELECT
            drivers_names.name_en
        FROM
            drivers_names
        ORDER BY random() LIMIT 1
    ) AS driver_name
FROM
    drivers

Result:

11  Denis
13  Denis
7   Denis

Tables structure.

drivers

+--------------+
| column_name  |
+--------------+
| driver_id    |
| property_1   |
| property_2   |
| property_3   |
+--------------+

drivers_names

+-------------+
| column_name |
+-------------+
| name_id     |
| name_en     |
+-------------+

Answer 1

Postgres probably evaluates the subselect only once because technically there’s no reason to evaluate it for every row.
You could force it by referencing a column from the drivers table into the subselect, like this:

SELECT
    drivers.driver_id AS drivers_driver_id,
    (
        SELECT
            drivers_names.name_en
        FROM
            drivers_names
        ORDER BY random()+drivers.driver_id LIMIT 1
    ) AS driver_name
FROM
    drivers