Re: UNIQUE boolean: only one row may be "true"

Hi Michael,

Per Bruno's suggestion, I think I will go with a partial index.  But 
your question brings up a good point and I am happy to hear what you 
have to say on the design.

The problem being solved is essentially replicating a very limited 
subset of SQL DDL to describe an external tabular dataset.  For a given 
dataset (thing_group) consisting of a number of columns (thing), there 
must be no more than one column which is a primary key 
(is_main_thing_p).  Do you have any suggestions on how this might 
better be implemented?  It seems to me that with a partial index, there 
is not too much overhead and it doesn't seem too offensively 
un-boycecoddean.

Best,

Randall

On Tuesday, May 6, 2003, at 01:13 PM, Michael Teter wrote:

> Can you elaborate a little on what you want to do with this (as in, 
> example
> data, problem domain, etc.)?
>
> This is probably not the answer you're looking for, but without 
> knowing more,
> I would guess that you need a different design, not a 1-true,many 
> false field
> constraint.
>
> Perhaps you want something like this?
>
> Table A:
> thing_groupid (primary key)
> ...other group info...
>
> Table B:
> thing_groupid (reference to A.thing_groupid, many to 1)
> thing_id
> ...details about thing...
>
> ?
>
> MT
>
> --- Randall Lucas <rlucas@tercent.net> wrote:
>>
>> Hi folks,
>>
>> After a late night of SQL hacking, my brain fizzed out:
>>
>> How shall I create a constraint such that only one row may be 'true'?
>> Rephrased, may I place a WHERE clause in a UNIQUE constraint, or
>> alternatively, may I use a CHECK constraint with an aggregate?
>>
>> Example:
>>
>> CREATE TABLE thing (
>>   thing_id             serial primary key,
>>   thing_group_id     int not null references thing_group(thing_group_id),
>>   is_main_thing_p     boolean not null default 'f',
>> -- there may be only one main thing per group:
>>   unique (thing_group_id, is_main_thing_p='t')
>> -- or else something like:
>> -- check (count (*) from thing where 
>> thing_group_id=NEW.thing_group_id 
>> and is_main_thing_p = 't' <2)
>> );
>>
>> Best,
>>
>> Randall
>>
>>
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