Question

I am quite new to triggers so obviously I am doing something wrong somewhere. I am working on a report table which will get the data from original tables. For the sake of simplicity, let’s say that there is one table and then there is one reporting table.

Original table (orig_tab)

CREATE TABLE orig_tab (
PK     NUMBER(8)       not null,
NAME   VARCHAR2(20)            ,
);

INSERT INTO orig_tab (PK, NAME) VALUES (1, 'AAA');
INSERT INTO orig_tab (PK, NAME) VALUES (2, 'BBB');
INSERT INTO orig_tab (PK, NAME) VALUES (3, 'CCC');

Then there is reporting table (rep_tab)

CREATE TABLE rep_tab (
PK     NUMBER(8)       not null,
NAME   VARCHAR2(20)            ,
);

Now from user inteface, someone changes the value of record 2. Obviously, this should be treated as an insert (because this record doesn’t exist) for reporting table. Then after sometime, the value is changed so it is an update case for reporting table.

Question: How may I make this kind of trigger? I assume that it is a merge statemement case.

This is what I have done:

create or replace trigger vr_reporting_trigger
after update on orig_tab
  for each row
begin
  MERGE INTO rep_tab d
  USING (SELECT pk FROM orig_tab) s
  ON (d.pk = s.pk)
  WHEN MATCHED THEN
  UPDATE SET d.pk = s.pk,
             d.name = s.name
  WHEN NOT MATCHED THEN
  INSERT (d.pk, d.name) VALUES (s.pk, s.name);
end vr_reporting_trigger;

Any suggestions or recommendations that can help me to figure it out? Thanks.

Answer 1

There are some corner cases that aren’t handled in previous answers.

What if a matching pk already exists in the reporting table, when a row is inserted. (We wouldn’t normally expect this to happen, but consider what would happen if someone deleted a row from the orig_tab, and then inserted it again. (This is the kind of problem that’s going to crop up in production, not in test, at the most inopportune time. Better to plan for it now.)

BEGIN
   IF inserting THEN
      -- insure we avoid duplicate key exception with a NOT EXISTS predicate
      INSERT INTO rep_tab(pk,name)
      SELECT :new.pk, :new.name FROM DUAL
      WHERE NOT EXISTS (SELECT 1 FROM rep_tab WHERE pk = :new.pk);
      -- if row already existed, there's a possibility that name does not match
      UPDATE rep_tab t SET t.name = :new.name 
       WHERE t.pk = :new.pk;
      -- could improve efficiency of update by checking if update is actually
      -- needed using a nullsafe comparison ( t.name <=> :new.name );
   ELSIF updating THEN
      -- handle updates to pk value (note: the row to be updated may not exist
      -- so we need to fallthru to the merge)
      IF :new.pk <> :old.pk THEN
         UPDATE rep_tab t
            SET t.pk = :new.pk
              , t.name = :new.name
          WHERE t.pk = :old.pk ;
      END IF;
      MERGE INTO rep_tab d
      USING DUAL ON (d.pk = :old.pk)
      WHEN MATCHED THEN
      UPDATE SET d.name = :new.name
      WHEN NOT MATCHED THEN
      INSERT (d.pk,d.name) VALUES (:new.pk,:new.name);
   END IF;
END;