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Asked: 2026-06-17T22:30:44+00:00

I have four tables. PERSON DELIVERY_MAPPING GENERATION_SYSTEM DELIVERY_METHOD —— —————- —————– ————— ID PERSON_ID

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I have four tables.

PERSON     DELIVERY_MAPPING       GENERATION_SYSTEM       DELIVERY_METHOD
------     ----------------       -----------------       ---------------
ID         PERSON_ID              ID                      ID
NAME       GENERATION_SYSTEM_ID   NAME                    NAME
                                  DELIVERY_METHOD_ID      IS_SPECIAL

Example data:

PERSON     DELIVERY_MAPPING       GENERATION_SYSTEM       DELIVERY_METHOD
------     ----------------       -----------------       ---------------
1. TOM       1    1               1. COLOR PRINTER 1      1. EMAIL    N
2. DICK      1    2               2. BW PRINTER    1      2. POST     N
3. HARRY     2    3               3. HANDWRITTEN   3      3. PIGEONS  Y

A DELIVERY_METHOD contains ways to deliver new letters — EMAIL, POST, PIGEON. The IS_SPECIAL column marks a record as a means of a special delivery. It is indicated by a simple value of Y or N. Only PIGEON is a special delivery method i.e. Y, the others are not i.e. N.

The GENERATION_SYSTEM has the information that will finally print the letter. Example values are COLOR PRINTER and DOT MATRIX PRINTER. Each GENERATION_SYSTEM will always be delivered using one of the DELIVERY_METHODs. There’s a foreign key-between GENERATION_SYSTEM and DELIVERY_METHOD.

Now, each PERSON can have his letters generated by different GENERATION_SYSTEMs and since, it is a many-to-many relation, we have the DELIVERY_MAPPING table and that’s that’s why we have foreign key’s on both ends.

So far, so good.

I need to ensure that it if a person has his letters generated by a system that uses a special delivery method then he cannot be allowed to have multiple generation systems in the mappings list. For example, Dick can’t have his letters generated using the colour printer because he already gets all his handwritten letters delivery by a pigeon (which is a marked a special delivery method).

How would I accomplish such a constraint? I tried doing it with a before-insert-or-update trigger on the DELIVERY_MAPPING table but that causes the mutating trigger problem when updating.

Can is normalise this scenario even more? Maybe it is just that i haven’t normalised my table properly.

Either way, I’d love to hear your take on this issue. I hope I’ve been verbose enough (…and if you can propose a better title for this post, that would be great)

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1 Answer

  1. Editorial Team
     CREATE MATERIALIZED VIEW special_queues_mv
  NOLOGGING
  CACHE
  BUILD IMMEDIATE 
  REFRESH ON COMMIT 
  ENABLE QUERY REWRITE
     AS SELECT dmap.person_id
             , SUM(DECODE(dmet.is_special, 'Y', 1, 0)) AS special_queues
             , SUM(DECODE(dmet.is_special, 'N', 1, 0)) AS regular_queues
          FROM delivery_mapping dmap
             , generation_system gsys
             , delivery_method dmet
         WHERE dmap.generation_system_id = gsys.id
           AND gsys.delevery_method_id = dmet.id
         GROUP
            BY dmap.person_id
/

  ALTER MATERIALIZED VIEW special_queues_mv
    ADD ( CONSTRAINT special_queues_mv_chk1 CHECK ((special_queues = 1 AND regular_queues = 0) OR ( regular_queues > 0 AND special_queues = 0 ) ) ENABLE VALIDATE)
/

    That’s how I did it. DazzaL’s answer gave me a hint on how to do it.

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