Oracle 18c

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create table teams as (select 'AAA' name, 'A' team_cod from dual union all
                       select 'BBB' name, 'B' team_cod from dual union all
                       select 'CCC' name, 'C' team_cod from dual union all
                       select 'DDD' name, 'D' team_cod from dual);
4 rows affected

create table matches as 
select 'A' team1, 'B' team2 from dual union all
select 'A' team1, 'C' team2 from dual union all
select 'D' team1, 'A' team2 from dual union all
select 'C' team1, 'A' team2 from dual;
4 rows affected

create table preleague as 
select 'A' team1, 'B' team2, 1 pts, 1 pts2 from dual union all
select 'A' team1, 'C' team2, 3 pts, 0 pts2 from dual union all
select 'D' team1, 'A' team2, 0 pts, 3 pts2 from dual union all
select 'C' team1, 'A' team2, 1 pts, 1 pts2 from dual;
4 rows affected

CREATE OR REPLACE FUNCTION leaguepts (team preleague.team1%TYPE) RETURN NUMBER AS
  cumulpts  NUMBER(3) := 0;
  cumulpts2 NUMBER(3) := 0;
  vt        matches.team1%TYPE;
  texist EXCEPTION;
BEGIN
  SELECT COUNT(team1)
  INTO   vt
  FROM   matches
  WHERE  team1 = team;

  IF (vt > 0)
  THEN
    SELECT SUM(pts), SUM(pts2)
    INTO   cumulpts, cumulpts2
    FROM   preleague
    WHERE  team2 = team;

  ELSE
    RAISE texist;
  END IF;

  RETURN nvl(cumulpts, 0) + nvl(cumulpts2, 0);

EXCEPTION
  WHEN texist THEN
    dbms_output.put_line ('The team "'||team||'" does not exist');
    return null;
  
END leaguepts;
/
CREATE OR REPLACE FORCE EDITIONABLE VIEW league (Team, PTS)   AS 
  SELECT name,
         leaguepts(team_cod) pts
  FROM   teams;
select * from league;


TEAM
PTS


AAA
5

BBB
null

CCC
3

DDD
0




dbms_output:
The team "B" does not exist