I have this table named ‘Details’ with 4 columns.

The requirement is if I insert a value in occupation column, I want this to insert on the first null row. Which means, I want the null rows of the column to be filled first.
For example,

insert into details(occupation)
values('student');

I want to create a function which will return the result like this if I just insert any new value

My question is, how to update the values of a column in a single query if the value is null then update with the new value.

I’ve come with these two function.

CREATE OR REPLACE FUNCTION update_null_value(value int)
RETURNS VOID AS $$
BEGIN
    UPDATE details
    SET occupation = value
    WHERE occupation IS NULL
    RETURNING occupation
    INTO value;

    SELECT value
    FROM details
    WHERE occupation= value
    LIMIT 1;
END;
$$ LANGUAGE plpgsql;

Whenver I pass a value, I get this error.
ERROR: query returned more than one row
HINT: Make sure the query returns a single row, or use LIMIT 1.
CONTEXT: PL/pgSQL function update_null_value(integer) line 3 at SQL statement
SQL state: P0003

If I limit 1 in the update, like this function. I get ERROR: syntax error at or near “limit” as we cant use limit 1 in update.

CREATE OR REPLACE FUNCTION update_null_value(value int)
RETURNS VOID AS $$
BEGIN
    INSERT INTO organization_map (organization_id)
    VALUES (value);

    UPDATE organization_map
    SET organization_id = value
    WHERE organization_id IS NULL
    limit 1
    RETURNING organization_id
    INTO value;

    SELECT value
    FROM organization_map
    WHERE organization_id = value
    order by organization_id
    LIMIT 1;
END;
$$ LANGUAGE plpgsql;

How to create a function the works for all the columns and rows.
Please help.