📍 1. 방법.

LENGTH를 이용.

LENGTH란
https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions076.htm
LENGTH 활용법
https://www.oracletutorial.com/oracle-string-functions/oracle-length/

📍 2. 코드

데이터 길이가 가장 긴 컬럼의 데이터 추출.

WITH TESTTABLE AS
(
    SELECT '123456789' AS NUMBERS FROM DUAL UNION ALL
    SELECT '12345678' AS NUMBERS FROM DUAL UNION ALL
    SELECT '1234567' AS NUMBERS FROM DUAL UNION ALL
    SELECT '123456' AS NUMBERS FROM DUAL UNION ALL
    SELECT '12345' AS NUMBERS FROM DUAL UNION ALL
    SELECT '1234' AS NUMBERS FROM DUAL UNION ALL
    SELECT '123' AS NUMBERS FROM DUAL UNION ALL
    SELECT '12' AS NUMBERS FROM DUAL UNION ALL
    SELECT '1' AS NUMBERS FROM DUAL    
)
SELECT NUMBERS
  FROM TESTTABLE
 WHERE LENGTH(NUMBERS) = (SELECT MAX(LENGTH(NUMBERS)) FROM TESTTABLE);

📍3. 결과