Oracle 21c

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ALTER SESSION SET NLS_TERRITORY = 'AMERICA';
SELECT TO_NUMBER('123.45', '999D99') FROM DUAL;

TO_NUMBER('123.45','999D99')

123.45

SELECT TO_NUMBER('123.45', '999.99') FROM DUAL;

TO_NUMBER('123.45','999.99')

123.45

SELECT TO_NUMBER('123.45', '999D99', 'NLS_NUMERIC_CHARACTERS=''.,''') AS value FROM DUAL;

VALUE

123.45

ALTER SESSION SET NLS_TERRITORY = 'PORTUGAL';
SELECT TO_NUMBER('123,45', '999D99') FROM DUAL;

TO_NUMBER('123,45','999D99')

123,45

SELECT TO_NUMBER('123.45', '999.99') FROM DUAL;

TO_NUMBER('123.45','999.99')

123,45

SELECT TO_NUMBER('123.45', '999D99', 'NLS_NUMERIC_CHARACTERS=''.,''') AS value FROM DUAL;

VALUE

123,45