Re: Check constraint and at least two rows

On Tue, Nov 3, 2015 at 7:09 PM, David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:

> On Tue, Nov 3, 2015 at 4:55 PM, Dane Foster <studdugie(at)gmail(dot)com> wrote:
>
>> Hello,
>>
>> I have a design/modelling puzzle/problem. I'm trying to model a series of
>> events. So I have two tables w/ a parent child relationship. The child
>> table has the rule/constraint/etc that for every row in the parent table
>> there must be at least 2 rows in the child because a series must have at
>> least 2 events to be a series.
>>
>> Now the SQL for the constraint is straight-forward and easy to write.
>> What I haven't figure out yet is where to put it because a straight forward
>> table constraint won't work because it's checked on every INSERT which
>> means it will be tripped on the first row inserted. A trigger doesn't seem
>> to fit the bill either.
>>
>> Ideas?
>>
>>
> ​http://www.postgresql.org/docs/devel/static/sql-createtrigger.html
> ​
>
> ​"""​
> In contrast, a trigger that is marked FOR EACH STATEMENT only executes
> once for any given operation, regardless of how many rows it modifies (in
> particular, an operation that modifies zero rows will still result in the
> execution of any applicable FOR EACH STATEMENT triggers
> ​.
> """
>
> ​That said while the "perfect" model may indeed conform to your definition
> as a practical matter what harm would there be in allowing zero or one
> child records for a given parent? Usually problems stem from designing a
> "zero-or-one" setup and then realizing that you actually have a "as many as
> you want" situation.​ Allowing a "as many as you want" setup to choose
> zero or one is significantly less problematic though you do need to be more
> aware of the need for LEFT JOINs.
>
David J.
>
> ​
I hear ya but ... what I'm working on is the migration of an existing
application from MySQL to PostgreSQL. And one of the things I've been
playing w/ recently is the mysql_fdw to do the migration of the data.​ I've
already encountered cases where implicit business rules have been violated
because there is no generalized constraint mechanism in MySQL (e.g., NOT
NULL doesn't count!) to make them explicit. So since PostgreSQL has a
generalized constraint mechanism (because NOT NULL doesn't get you very
far) I'd like to take advantage of it and eliminate the possibility of
developers screwing up my data model and data. Because I have the luxury of
knowing what ALL the requirements are I don't have to play the "I need to
be flexible" game. The game I want to play is, "ha you can't introduced
logical inconsistencies into my data model!"

Dane
​