From: | Mark Lybarger <mlybarger(at)gmail(dot)com> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Subject: | unique constraint with several null values |
Date: | 2016-07-20 18:14:00 |
Message-ID: | CAKM-gm+_boZfqLrfmZz1Z2=iNTB68nj_H=qJLt3bJjwmUrVnjg@mail.gmail.com |
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Lists: | pgsql-general |
I have a relation such as
create table order_item ( id uuid not null primary key, order_id number not
null, item_code text, make text, model text, reason text, size text,
expiration_date timestamp );
where the combination of the columns order_id, item_code, make, model,
reason, size must be unique (unless there's an expiration date).
I'm inclined to use a unique index:
create unique index unique_index_order_item_1 on order_item (order_id,
item_code, make, model, reason, size)
where expiration_date is null;
this works as expected and the duplicate row is rejected
:
insert into order_item ( 1, 88, 'CODE', 'MAKE','MODEL','REASON', 'SIZE',
null); <- first adds
insert into order_item ( 1, 88, 'CODE', 'MAKE','MODEL','REASON', 'SIZE',
null); <- rejects
however, nulls are allowed for all the columns except the order_id. so,
when I add a null value, it fails to meet my expectations,
insert into order_item ( 1, 88, 'CODE', 'MAKE',null,'REASON', 'SIZE',
null); <- first adds
insert into order_item ( 1, 88, 'CODE', 'MAKE',null,'REASON', 'SIZE',
null); <- adds, but should reject.
This leads me to think I need to create 2^5 or 32 unique constraints to
handle the various combinations of data that I can store. Until now, this
integrity is handled in the application code. That breaks when the
application is multi-threaded and the rules are not applied at the database
level.
Another solution I can think of is to just use a trigger to prevent the
duplicate rows.
Any thoughts are certainly appreciated. I can't do much about the data
model itself right now, I need to protect the integrity of the data.
Thanks!
-mark-
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