WITH t(v) AS
(
	SELECT 'image,search,photo,image,photo' FROM dual
)

SELECT LISTAGG(v, ',') WITHIN GROUP(ORDER BY lv, rn) v
FROM 
(
	SELECT v
	     , lv
	     , ROW_NUMBER() OVER(PARTITION BY v ORDER BY lv) rn
	FROM 
	(
		SELECT REGEXP_SUBSTR(t.v, '[^,]+', 1, LEVEL) v, LEVEL lv
		FROM t
		CONNECT BY LEVEL <= REGEXP_COUNT(t.v, ',') + 1
	)
)
WHERE rn = 1

MySQL

WITH a (id, col) AS (
    SELECT 1, 'image,search,photo,image,photo' UNION ALL
    SELECT 2, 'a,a,b,c,c'
), t (seq) AS (
    SELECT 0 AS seq UNION ALL
    SELECT 1 UNION ALL
    SELECT 2 UNION ALL
    SELECT 3 UNION ALL
    SELECT 4 UNION ALL
    SELECT 5 UNION ALL
    SELECT 6 UNION ALL
    SELECT 7 UNION ALL
    SELECT 8 UNION ALL
    SELECT 9
)
SELECT a.id, GROUP_CONCAT(DISTINCT SUBSTRING_INDEX(SUBSTRING_INDEX(a.col, ',', t.seq + 1), ',', -1))
FROM a
    INNER JOIN t ON t.seq <= LENGTH(a.col) - LENGTH(REPLACE(a.col, ',', ''))
GROUP BY a.id;

MS-SQL 2017

WITH a (id, col) AS (
    SELECT 1, 'image,search,photo,image,photo' UNION ALL
    SELECT 2, 'a,a,b,c,c'
), b (id, col) AS (
	SELECT a.id, t.value
	FROM a
		CROSS APPLY STRING_SPLIT(a.col, ',') t
	GROUP BY a.id, t.value
)
SELECT id, STRING_AGG(col, ',')
FROM b
GROUP BY id;

두분다 감사합니다.

제가 확인하는 환경이 오라클인지라 랑에1님 가이드대로 적용해서 원하는 결과를 얻었습니다.

감사합니다~