SQL> ed
Wrote file afiedt.buf

  1  CREATE OR REPLACE TRIGGER students_auid
  2  AFTER INSERT OR UPDATE OR DELETE ON students FOR EACH ROW
  3  BEGIN
  4  IF   INSERTING then
  5    update stats set kol=kol+1 where fo=:new.fo;
  6  ELSIF DELETING then
  7    update stats set kol=kol-1 where fo=:old.fo;
  8  ELSIF UPDATING and :new.fo<>:old.fo then
  9    update stats set kol=kol+1 where fo=:new.fo ; --or но тогда надо order by
 10    update stats set kol=kol-1 where fo=:old.fo ;
 11  END IF;
 12* END;
SQL> /

Trigger created.

SQL> select * from stats;

FO         KOL
--- ----------
o            0
z            0
oz           0

SQL> insert  into students values(1,'Stax','o');

1 row created.

SQL> insert  into students values(2,'Bastist29','z');

1 row created.

SQL> select * from stats;

FO         KOL
--- ----------
o            1
z            1
oz           0

SQL> update students set fo='oz' where tabn=1;

1 row updated.

SQL> select * from stats;

FO         KOL
--- ----------
o            0
z            1
oz           1

SQL> select * from students;

      TABN FIO                  FO
---------- -------------------- ---
         1 Stax                 oz
         2 Bastist29            z

SQL> delete students where tabn=2;

1 row deleted.

SQL> select * from students;

      TABN FIO                  FO
---------- -------------------- ---
         1 Stax                 oz

SQL> select * from stats;

FO         KOL
--- ----------
o            0
z            0
oz           1

SQL>