Re: Quasi Foreign Key

In article <8tmqrt$bl0$1_at_bob.news.rcn.net>,   "Steve Long" <steven.long_at_erols.com> wrote:
> i have the following scenario and could use some assistance.
>
> parent table has composite primary key consisting of three columns.
>
> child table has a column which has a foreign key to parent table
 using the
> first column of the parent table's composite primary key. however,
 the
> child table's column may be null, but if it is not null it's value
 must be
> present in the parent table's column.
>
> the problem is that the value "null" in the child table's column is
 not an
> allowable value in the parent table's column since in the parent
 table this
> column is part of the primary key.
>
> any suggestions on how to implement this?
>
>

You are correct in refering to this as a 'quasi-foreign key' and declaring it as a foreign key will not work as there are allowable null values in the column. I would suggest a trigger to validate non-null values in the 'child' table against the 'parent' table. If the column value is null then the trigger would perform no task; if the column value is not null the value would be checked against the valid entries in the 'parent' table to determine if the value .

--
David Fitzjarrell
Oracle Certified DBA


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