Вот наиболее универсальное решение для заданного вопроса:
CREATE OR REPLACE FUNCTION NUM_TO_CHAR(p_number IN NUMBER,
p_decimal_places IN NUMBER DEFAULT 3)
RETURN VARCHAR2 IS
p_number2 NUMBER;
p_number3 NUMBER;
p_mask VARCHAR2(1000):='0';
BEGIN
p_number3:=ABS(ROUND(p_number,0));
IF (p_number3>9) THEN
FOR i IN 1..(LENGTH(TO_CHAR(p_number3))-1)
LOOP
p_mask := '9'||p_mask;
END LOOP;
END IF;
p_number2:=ROUND(p_number,p_decimal_places);
IF (p_decimal_places>0) THEN
p_mask:=p_mask||'.';
FOR i IN 1..p_decimal_places
LOOP
p_mask := p_mask||'9';
END LOOP;
END IF;
RETURN trim(to_char(p_number2, p_mask));
EXCEPTION
WHEN OTHERS THEN
RETURN '0';
END NUM_TO_CHAR;
/