Maybe you could do it without updating!?
I would probably try to :

CREATE NEW_TABLE AS 
    SELECT [do the "update" here] FROM OLD_TABLE;

- add indexes on new table
- add constraints on new table
- add anything else you need on new table (foreign keys, grants...)

and then

DROP TABLE OLD_TABLE; 
--  and 
RENAME NEW_TABLE To OLD_TABLE;

SELECT with your sample data:

WITH
    tbl as
        (
            Select 1 "NUM", 9234 "STUDENT_ID" From Dual Union All
            Select 2 "NUM", 9234 "STUDENT_ID" From Dual Union All
            Select 3 "NUM", 9234 "STUDENT_ID" From Dual Union All
            Select 4 "NUM", 3456 "STUDENT_ID" From Dual Union All
            Select 5 "NUM", 3456 "STUDENT_ID" From Dual Union All
            Select 6 "NUM", 789  "STUDENT_ID" From Dual Union All
            Select 7 "NUM", 956  "STUDENT_ID" From Dual  
        )
Select
    NUM, 
    CASE WHEN Count(NUM) Over(Partition By STUDENT_ID) = 1 THEN 'SS' || STUDENT_ID 
    ELSE 'SS' || Replace(Sum(1) Over(Partition By STUDENT_ID Order By NUM) - 1, 0, '') || STUDENT_ID
  END "STUDENT_ID"
From    
    tbl
Order By NUM

Result:
| NUM | STUDENT_ID |
| ------------ | ------------ |
| 1 | SS9234 |
| 2 | SS19234 |
| 3 | SS29234 |
| 4 | SS3456 |
| 5 | SS13456 |
| 6 | SS789 |
| 7 | SS956 |

You can use a MERGE statement correlated on the ROWID pseudo-column and using the ROW_NUMBER() analytic function:

MERGE INTO table_name dst
USING (
  SELECT ROWID as rid,
         ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY num) AS rn
  FROM   table_name
) src
ON (src.rid = dst.ROWID)
WHEN MATCHED THEN
  UPDATE
  SET student_id = 'SS' || CASE WHEN rn > 1 THEN rn - 1 END || dst.student_id;

Which, for the sample data:

CREATE TABLE table_name (NUM, STUDENT_ID) AS
SELECT 1, CAST('9234' AS VARCHAR2(20)) FROM DUAL UNION ALL
SELECT 2, '9234' FROM DUAL UNION ALL
SELECT 3, '9234' FROM DUAL UNION ALL
SELECT 4, '3456' FROM DUAL UNION ALL
SELECT 5, '3456' FROM DUAL UNION ALL
SELECT 6, '789'  FROM DUAL UNION ALL
SELECT 7, '956'  FROM DUAL;

Then after the MERGE the table contains:
| NUM | STUDENT_ID |
| ------------ | ------------ |
| 1 | SS9234 |
| 2 | SS19234 |
| 3 | SS29234 |
| 4 | SS3456 |
| 5 | SS13456 |
| 6 | SS789 |
| 7 | SS956 |
fiddle

我确信一定有更好的方法，但下面的查询可以完成这项工作：

update t
set student_id = (
  select new_student_id
  from (
    select x.*, 'SS' || case when rn = 1 then '' else '' || rn end 
      || student_id as new_student_id
    from (
      select t.*, row_number() over(partition by student_id order by num) as rn
      from t
    ) x
  ) y
  where t.num = y.num
)

结果：

NUM  STUDENT_ID 
 ---- ---------- 
 1    SS9234     
 2    SS29234    
 3    SS39234    
 4    SS3456     
 5    SS23456    
 6    SS789      
 7    SS956

请参阅db<>fiddle上的运行示例。