I was completing a HackerRank question on symmetric pairs and succeeded with the following SQL query (Oracle 11g):

SELECT
    DISTINCT F1.X, F1.Y
FROM
    (
        SELECT
            X, Y, ROWNUM AS ROW_NUM
        FROM
            Functions
    ) F1,
    (
        SELECT
            X, Y, ROWNUM AS ROW_NUM
        FROM
            Functions
    ) F2
WHERE
    F1.X = F2.Y
    AND F2.X = F1.Y
    AND F1.X <= F1.Y
    AND F1.ROW_NUM != F2.ROW_NUM
ORDER BY
    F1.X;

However, when I try to simplify it with the following query:

SELECT
    DISTINCT F1.X, F1.Y
FROM
    Functions F1,
    Functions F2
WHERE
    F1.X = F2.Y
    AND F2.X = F1.Y
    AND F1.X <= F1.Y
    AND F1.ROWNUM != F2.ROWNUM
ORDER BY
    F1.X;

I received the following error:

ORA-01747: invalid user.table.column, table.column, or column specification

Why doesn’t this work? Why do I have to give an alias to ROWNUM?