4. Save it. It shows error as Serial_n column is null.

1. "max + 1" principle works fine (if correctly set - which you didn't) in a single-user environment. Otherwise, it might cause DUP-VAL-ON-INDEX error (if two or more users try to insert the same "max + 1" value into a column which is designed as unique). For unique values, use Oracle object called sequence
   
2. Your "max + 1" doesn't work correctly. Here's why, based on a simple example in Scott's schema (there's no department in the "dept" table whose location is "Zagreb"):

   SQL> select * from dept where loc = 'Zagreb';
   
   no rows selected

   
   Your query: the result is NULL (this is what you are trying to insert into the "serial_n" column):

   SQL> select max(nvl(deptno, 0)) + 1 from dept where loc = 'Zagreb';
   
   MAX(NVL(DEPTNO,0))+1
   --------------------
   
   
   SQL>

   
   Another way:

   SQL> select nvl(max(deptno), 0) + 1 from dept where loc = 'Zagreb';
   
   NVL(MAX(DEPTNO),0)+1
   --------------------
                      1
   
   SQL>