Array values and foreign keys

[Daniel Savard]

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

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[Pierre-Frédéric Caillaud]

Re: Array values and foreign keys



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.c a> wrote:
[color=blue]
> 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@postg resql.org
>[/color]



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[Michael Fuhr]

Re: Array values and foreign keys

On Wed, Oct 27, 2004 at 05:59:46PM +0200, Pierre-Fr?d?ric Caillaud wrote:[color=blue]
>
> - 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[/color]

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/

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[Daniel Savard]

Re: Array values and foreign keys

Le mer 27/10/2004 à 11:59, Pierre-Frédéric Caillaud a écrit :[color=blue]
>
>
> 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.
>
> [/color]

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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