Re: conditional constraints

Also, (I would love to be corrected nicely if I'm wrong), a column can be
NULL'able and have a foreign contraint on it, and have NULL values, right?

Jan Wieck wrote:
> tom dyson wrote:
>
>>(on behalf of my colleague, Neal Todd)
>>
>>This question is about whether it's possible to have conditionality on a
>>constraint, or rather (presuming it's not possible), how it can be emulated
>>perhaps with a trigger.
>>
>>The scenario is this (but is fairly general anyway)...
>>
>>Table "P" storing projects with a project id primary key.
>>and
>>Table "D" storing diary entries relating to projects with foreign key
>>constraint referencing project ids in table "P".
>>
>>Fine so far, we have referential integrity on the project ids in table "D".
>>
>>However, we need to add diary entries that are for a generic "non-project"
>>category. Without the constraint we could just have a null or dummy (e.g. 0)
>>entry in D's project id foreign key. But with the constraint the referential
>>integrity is broken.
>
>
> Your assumption is wrong. That's the strange thing about NULL values, we
> can't tell what they are made of or where they are coming from, but we
> certainly know what they are good for :-)
>
>
> Jan
>