Re: OOP real life example (was Re: Why is MySQL more

On Mon, 2002-08-12 at 15:00, Greg Copeland wrote:
...
> Look a little deeper here.  In other OO implementations, I can define a
> class (say class a) which has no instances (abstract base class). 
> Furthermore, I can take this case and use it for building blocks
> (assuming multiple inheritance is allowed in this world) by combining
> with other classes (z inherits from a, b, c; whereby classes a, b, c
> still do not have an actual instance).  I can create an instance of my
> newly inherited class (z).
> 
> Seems to me that there is some distinction between types (classes) and
> and type instances (instance of a specific class) as it pertains to it's
> usability.
> 
> How exactly would you create an abstract base class for table type?

CREATE TABLE abstract_base (
   cols ...,
   CONSTRAINT "No data allowed in table abstract_base!" CHECK (1 = 0)
)

This assumes that the constraint is not inherited or can be removed in
child tables.

-- 
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight, UK                            
http://www.lfix.co.uk/oliver
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