Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE t
    ("S_NUM" int, "EFF_DATE" date, "AMOUNT" int)
;

ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY';
INSERT ALL 
    INTO t ("S_NUM", "EFF_DATE", "AMOUNT")
         VALUES (1, '07-Oct-2018', 110)
    INTO t ("S_NUM", "EFF_DATE", "AMOUNT")
         VALUES (1, '06-Oct-2018', 50)
    INTO t ("S_NUM", "EFF_DATE", "AMOUNT")
         VALUES (2, '07-Oct-2018', 200)
    INTO t ("S_NUM", "EFF_DATE", "AMOUNT")
         VALUES (2, '06-Oct-2018', 150)
SELECT * FROM dual
;
4 rows affected

SELECT s_num,SUM (CASE 
              WHEN eff_date >= trunc(sysdate) 
                   AND eff_date < TRUNC(sysdate) + 1 THEN amount 
              WHEN eff_date >= trunc(sysdate) - 1 
                   AND eff_date < TRUNC(sysdate) THEN - amount 
            END) as diff
FROM   t 
GROUP  BY s_num; 


S_NUM
DIFF


1
60

2
50


SELECT s_num, 
       diff 
FROM   (SELECT s_num, 
               eff_date,
               amount - LAG(amount, 1, 0) 
                          OVER ( 
                            partition BY s_num 
                            ORDER BY eff_date ) AS diff 
        FROM   t) 
WHERE  eff_date >= trunc(sysdate) 
       AND eff_date < trunc(sysdate) + 1 ;


S_NUM
DIFF


1
60

2
50