SELECT 
  REPORT_MONTH, 
  CATEGORY_A, 
  NVL(CATEGORY_A, LAG(CATEGORY_A, 1) OVER (ORDER BY REPORT_MONTH) * ROUND(CATEGORY_B / LAG(CATEGORY_B, 1) OVER (ORDER BY REPORT_MONTH), 2)) ADJUSTED_CATEGORY_A,
  CATEGORY_B, 
  ROUND(CATEGORY_B / LAG(CATEGORY_B, 1) OVER (ORDER BY REPORT_MONTH), 2) CATEGORY_B_CHANGE_RATIO
FROM (
  SELECT TO_DATE('01-JAN-2016') REPORT_MONTH, 100 CATEGORY_A, 200 CATEGORY_B FROM DUAL
  UNION ALL
  SELECT TO_DATE('01-FEB-2016') REPORT_MONTH, NULL CATEGORY_A, 400 CATEGORY_B FROM DUAL
  UNION ALL
  SELECT TO_DATE('01-MAR-2016') REPORT_MONTH, NULL CATEGORY_A, 200 CATEGORY_B FROM DUAL
  UNION ALL
  SELECT TO_DATE('01-APR-2016') REPORT_MONTH, NULL CATEGORY_A, 100 CATEGORY_B FROM DUAL
  UNION ALL
  SELECT TO_DATE('01-MAY-2016') REPORT_MONTH, NULL CATEGORY_A, 200 CATEGORY_B FROM DUAL
  UNION ALL
  SELECT TO_DATE('01-JUN-2016') REPORT_MONTH, NULL CATEGORY_A, 100 CATEGORY_B FROM DUAL
)