Oracle 11g Release 2

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CREATE TABLE tableA
    ("ID" int, "COUNTER" int, "GID_FROM_B" varchar2(3))
;
INSERT ALL 
    INTO tableA ("ID", "COUNTER", "GID_FROM_B")
         VALUES (1, 3, 'xyz')
SELECT * FROM dual
;
1 rows affected

CREATE TABLE tableB
    ("ID" int, "COUNTER" int, "GID" varchar2(3))
;
INSERT ALL 
    INTO tableB ("ID", "COUNTER", "GID")
         VALUES (1, 3, 'xyz')
    INTO tableB ("ID", "COUNTER", "GID")
         VALUES (2, 5, 'abc')
SELECT * FROM dual
;
2 rows affected

CREATE OR REPLACE TRIGGER gid_update
   BEFORE INSERT OR UPDATE ON TABLEA
FOR EACH ROW WHEN (NEW.gid_from_b IS NULL)
BEGIN
 SELECT gid INTO 
    :NEW.gid_from_b FROM tableB b 
  WHERE b.id =:NEW.id AND b.counter = :NEW.counter;
END;
/
INSERT INTO TableA (id,counter) VALUES( 2,5);
1 rows affected

select * FROM TableA;


ID
COUNTER
GID_FROM_B


1
3
xyz

2
5
abc


INSERT INTO TableA (id,counter,gid_from_b) VALUES( 1,3,'def');
1 rows affected

select * FROM TableA;


ID
COUNTER
GID_FROM_B


1
3
xyz

2
5
abc

1
3
def