Thread: Array values and foreign keys

Is there a way to define a foreign key for the values of an array?

For example, if table T1 is having a colum A which is defined as integer[] can I define a foreign key in order to force
eachvalue to be a pointer (index) to a row in a table T2? 

If yes, how? Is there any shortcomings to this approach?

Thanks,

Daniel Savard

---
Daniel Savard
dsavard@cids.ca

    You can't express it directly with a CHECK constraint but you can do this
:

    - add CHECK( test_array( yourcolumn )) in your table definition
    - create function test_array which takes an array and looks if all its
elements are in your table T2, I do something like comparing the length of
the array to SELECT count(1) FROM T2 WHERE key IN array
    You can do it other ways but you'll have to use a function.


On Wed, 27 Oct 2004 10:19:02 -0400, Daniel Savard <dsavard@cids.ca> wrote:

> Is there a way to define a foreign key for the values of an array?
>
> For example, if table T1 is having a colum A which is defined as
> integer[] can I define a foreign key in order to force each value to be
> a pointer (index) to a row in a table T2?
>
> If yes, how? Is there any shortcomings to this approach?
>
> Thanks,
>
> Daniel Savard
>
> ---
> Daniel Savard
> dsavard@cids.ca
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
>

On Wed, Oct 27, 2004 at 05:59:46PM +0200, Pierre-Fr?d?ric Caillaud wrote:
>
>     - add CHECK( test_array( yourcolumn )) in your table definition
>     - create function test_array which takes an array and looks if all
>     its  elements are in your table T2, I do something like comparing the
> length of  the array to SELECT count(1) FROM T2 WHERE key IN array

This provides only partial foreign key checking: depending on how
the application works, you might also need to ensure that updates
and deletes in T2 don't break the references in T1.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/

Le mer 27/10/2004 à 11:59, Pierre-Frédéric Caillaud a écrit :
>
>
>     You can't express it directly with a CHECK constraint but you can do this
> :
>
>     - add CHECK( test_array( yourcolumn )) in your table definition
>     - create function test_array which takes an array and looks if all its
> elements are in your table T2, I do something like comparing the length of
> the array to SELECT count(1) FROM T2 WHERE key IN array
>     You can do it other ways but you'll have to use a function.
>
>

Fine. I got it right after fiddling a little bit. The function is
something like:

CREATE FUNCTION test_array (smallint[]) RETURNS bool AS '
   select case when count(1) = array_upper($1,1) then true
               else false
          end from t2 where cle = any($1);
' LANGUAGE SQL;

It compares the length of the array to the number of elements actually
found in the reference table.

--

=======================================
Daniel Savard

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