ORACLE XMLAGG

--가상 테이블
WITH TEST_TABLE AS (
    SELECT '고구려' COUNTRY, '1대' ST, '동명성왕'   KING_NM FROM DUAL UNION ALL
    SELECT '고구려' COUNTRY, '3대' ST, '대무신왕'   KING_NM FROM DUAL UNION ALL
    SELECT '백제'   COUNTRY, '1대' ST, '온조왕'     KING_NM FROM DUAL UNION ALL
    SELECT '고구려' COUNTRY, '2대' ST, '유리왕'     KING_NM FROM DUAL UNION ALL
    SELECT '백제'   COUNTRY, '3대' ST, '기루왕'     KING_NM FROM DUAL UNION ALL
    SELECT '신라'   COUNTRY, '2대' ST, '남해왕'     KING_NM FROM DUAL UNION ALL
    SELECT '신라'   COUNTRY, '1대' ST, '박혁거세'   KING_NM FROM DUAL UNION ALL
    SELECT '백제'   COUNTRY, '2대' ST, '다루왕'     KING_NM FROM DUAL UNION ALL
    SELECT '신라'   COUNTRY, '3대' ST, '유리이사금' KING_NM FROM DUAL
)

--조회 쿼리
SELECT COUNTRY
     , SUBSTR(
              XMLAGG(
                     XMLELEMENT(COL ,',', KING_NM) ORDER BY ST
                    ).EXTRACT('//text()').GETSTRINGVAL()
          , 2) KING_NM
  FROM TEST_TABLE
 GROUP BY COUNTRY