在 Postgres 触发器创建中出错

Question

I'm trying to create this trigger in my PostgresSql environment:

CREATE TRIGGER MYTRIGGER
BEFORE INSERT
ON MYTABLE
FOR EACH ROW
BEGIN
  IF( LENGTH( :NEW.VAL ) > 10 )
  THEN
    RAISE_APPLICATION_ERROR( -20003, 
                             'Cannot exceed 10 chars' );
  END IF;

  IF :NEW.FQN_ID IS NULL THEN
  :NEW.FQN_ID :=
      CASE :NEW.SUBTYPECODE
        WHEN NULL  THEN 'A:'
        WHEN 0     THEN 'B:'
        WHEN 1     THEN 'C:'
        WHEN 2     THEN 'D:'
      ELSE 'Z:' || :NEW.SUBTYPECODE || '::'
      --END || :NEW.OBJECTID;
      END || STRUCTURE_FQNID_SEQ.NEXTVAL;
 END IF;
END;

But I get this error:

 ERROR: syntax error at or near "BEGIN" LINE 5: BEGIN ^ SQL state: 42601 Character: 79

I think I'm missing something but I can't get it.

Any suggestion would be greatly appreciated.

Thank you.

Answer 1

Here are my notes about triggers in several DBMSs: https://github.com/iwis/SQL-notes . I marked the differences between the DBMSs in orange. I think that the notes are quite complete, so you don't have to read about triggers in Postgres documentation.

I see the following changes that need to done in your example:

• Change Oracle :NEW to Postgres NEW .
• Instead of a BEGIN... END block, write EXECUTE FUNCTION my_trigger_function(); , where my_trigger_function is a function that needs to be created like in the example given by a_horse_with_no_name. This function should return NEW in your case - the reason is described here .

If a more complicated code is fired by a trigger, then you also need to understand the differences between PL/SQL and PL/pgSQL languages. These languages are quite similar, though there are some differences. Your code is simple so the differences are small. It's probably enough to:

• Write Postgres RETURNS in the function definition instead of Oracle RETURN .
• Write $$ before BEGIN , and $$ LANGUAGE plpgsql; after END .
• Write Postgres RAISE 'Cannot exceed 10 chars'; instead of Oracle RAISE_APPLICATION_ERROR(-20003, 'Cannot exceed 10 chars'); .

I don't know if sequences work in the same way in PostgreSQL - I haven't read about them yet.

Let me know if my notes are understandable - I'm not sure about it because they are super compact so you need to decipher the markings used by me.