From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200301310532.h0V5W4G11349@candle.pha.pa.us>
Subject: Re: checking data integrity in a recursive table
In-Reply-To: <019801c2c31c$c8d63fa0$3200000a@K2>
To: Luke Pascoe <luke.p@kmg.co.nz>
Date: Fri, 31 Jan 2003 00:32:04 -0500 (EST)
Cc: pgsql-sql@postgresql.org
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Luke Pascoe wrote:
> Hi, I have a table that references itself to create a tree-like structure,
> eg:
>     CREATE TABLE tree (
>         id     SERIAL NOT NULL,
>         name   VARCHAR(255) NOT NULL,
>         parent INT NULL,
>         customer IN NOT NULL,
>         CONSTRAINT parent_key...
>         CONSTRAINT customer_fk FOREIGN KEY (customer) REFERENCES customer
>         );
>     ALTER TABLE tree ADD CONSTRAINT FOREIGN KEY (parent) REFERENCES tree;
> 
> As you can see tree also references the customer table.
> 
> What I need is a CHECK that will ensuer that any given "tree" row has the
> same customer as its parent.
> Remember that "parent" can also be NULL.
> 
> Or would this be better done as a trigger?

Good question.  I don't think you can do actualy SQL lookups in a CHECK.
I think you will need a trigger, either in pl/pgsql or in C using SPI to
issue the lookup queries.

You can have a CHECK clause that deals with multiple columns:

	CREATE TABLE friend2 (
	             firstname CHAR(15),
	             lastname  CHAR(20),
	             city      CHAR(15),
	             state     CHAR(2)      CHECK (length(trim(state)) = 2),
	             age       INTEGER      CHECK (age >= 0),
	             gender    CHAR(1)      CHECK (gender IN ('M','F')),
	             last_met  DATE         CHECK (last_met BETWEEN '1950-01-01'
	                                           AND CURRENT_DATE),
	             CHECK (upper(trim(firstname)) != 'ED' OR
	                    upper(trim(lastname)) != 'RIVERS')
	);

However, that doesn't help you because you can't reference a column in
another row of the same table.


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