WITH T(EX, A, B) AS (
SELECT '감자'  , TO_DATE('20210511 114000', 'YYYYMMDD HH24MISS'), 50 FROM DUAL UNION ALL
SELECT '고구마', TO_DATE('20210511 113000', 'YYYYMMDD HH24MISS'), 100 FROM DUAL
)
SELECT ABS( MAX( CASE WHEN EX = '감자'   THEN A ELSE NULL END )
          - MAX( CASE WHEN EX = '고구마' THEN A ELSE NULL END ) ) * 24 * 60 AS DIFF_A_MIN
  FROM T
 WHERE 1 = 1

아래처럼 구하긴 했는데.. 이런방식을 원하시는게 맞는지요?

with t (ex, a, b) as (
select '감자'  , '20210511 11:40:00',  50 from dual union all
select '고구마', '20210511 11:30:00', 100 from dual
)
select to_timestamp(max(case when ex='감자' then a end)  , 'YYYYMMDD HH:MI:SS') - 
       to_timestamp(max(case when ex='고구마' then a end), 'YYYYMMDD HH:MI:SS')
from t

SELECT (MAX(a) - MIN(a)) * 24 * 60 minute
  FROM t
;