Oracle 11g Release 2

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CREATE TABLE sectores (
  codS NUMBER(2),
  nombreS VARCHAR2(20),
  porcentS NUMBER(5,2),
  ingresosS NUMBER(9,2),
  num_pob NUMBER(3),
  PRIMARY KEY (codS)
);

CREATE TABLE poblacion (
  dni VARCHAR2(9), 
  nombre VARCHAR2(12), 
  apellido1 VARCHAR2(12), 
  apellido2 VARCHAR2(12),
  fechanac DATE,
  direccion VARCHAR2(20),
  cp VARCHAR2(5),
  sexo VARCHAR2(1),
  ingresos NUMBER(7,2),
  gastosFijos NUMBER(7,2), 
  gastosAlim NUMBER(7,2), 
  gastosRopa NUMBER(7,2), 
  sector NUMBER(2), 
  PRIMARY KEY (dni),
  FOREIGN KEY (sector) REFERENCES sectores (codS) ON DELETE SET NULL
);
CREATE OR REPLACE TRIGGER ingresosS1 
AFTER INSERT ON poblacion 
REFERENCING NEW AS NUEVA
FOR EACH ROW
WHEN (NUEVA.sector IS NOT NULL) 
BEGIN
  UPDATE sectores
  SET ingresosS = ingresosS + :NUEVA.ingresos
  WHERE codS = :NUEVA.sector;

  UPDATE sectores
   SET num_pob = COALESCE(num_pob,0) + 1 
  WHERE codS = :NUEVA.sector;

  UPDATE sectores 
  SET porcentS = COALESCE(100 * ingresosS / (SELECT SUM(ingresosS) FROM sectores),0);
END;
/
CREATE OR REPLACE TRIGGER muchaGente
AFTER INSERT OR UPDATE ON sectores
FOR EACH ROW
WHEN (NEW.codS IS NOT NULL)
DECLARE 
    counter INTEGER;
    numPOB INTEGER;
BEGIN
   counter  := :OLD.num_pob;
    numPOB  := counter - 1;
    IF counter > 3
        THEN 
     RAISE_APPLICATION_ERROR( -20001, 
                             'There are more than 3 persons in the same Sector' );
    END IF;
END;
/
INSERT INTO sectores(codS) VALUES (10);
1 rows affected

INSERt INTO poblacion(dni,sector) VALUES(1,10);
1 rows affected

INSERt INTO poblacion(dni,sector) VALUES(2,10);
1 rows affected

INSERt INTO poblacion(dni,sector) VALUES(3,10);
1 rows affected

INSERt INTO poblacion(dni,sector) VALUES(4,10);
ORA-20001: There are more than 3 persons in the same Sector
ORA-06512: at "FIDDLE_DHMINEUQLEBBEFSQGRSM.MUCHAGENTE", line 9
ORA-04088: error during execution of trigger 'FIDDLE_DHMINEUQLEBBEFSQGRSM.MUCHAGENTE'
ORA-06512: at "FIDDLE_DHMINEUQLEBBEFSQGRSM.INGRESOSS1", line 10
ORA-04088: error during execution of trigger 'FIDDLE_DHMINEUQLEBBEFSQGRSM.INGRESOSS1'

select * from sectores;


CODS
NOMBRES
PORCENTS
INGRESOSS
NUM_POB


10
null
0
null
3