MERGE INTO uf_AttendanceList_dt1 a
USING (
    SELECT 
        a1.id,
        LEAD(TO_DATE(a1.EFF_DATE, 'YYYY-MM-DD')) 
            OVER (PARTITION BY a1.mainid ORDER BY TO_DATE(a1.EFF_DATE, 'YYYY-MM-DD')) AS NEXT_EFF_DATE
    FROM uf_AttendanceList_dt1 a1
) t
ON (a.id = t.id)
WHEN MATCHED THEN
    UPDATE SET a.EXP_DATE = TO_CHAR(t.NEXT_EFF_DATE - 1, 'YYYY-MM-DD')

    WHERE t.NEXT_EFF_DATE IS NOT NULL;