Hi Miguel.

Very strange. Where did you typed "select '​abc' as emm from dual"? Your 'abc' has a hidden character just before a.

I execute:

SELECT
    '1st' descr,
    emm
FROM
    (
        SELECT
            '​abc' emm
        FROM
            dual
    ) t
WHERE
    emm = 'abc'
UNION
SELECT
    '2nd'   descr,
    '​abc'  emm
FROM
    dual
ORDER BY
    emm

This strangely returns '?abc'...

Andreas.