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Weird (?) problem with order of conditions in SELECT

From: "Mark, Terry" <tmark(at)amgen(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Weird (?) problem with order of conditions in SELECT
Date: 2001-03-20 16:06:02
Message-ID: 548152BB0AD9D2119C400008C7CFE8C805AFCBD3@gold-exch.amgen.com (view raw or flat)
Thread:
Lists: pgsql-novice
Hello all,

I have encountered a weird problem I  can't seem to understand.  It involves
a correlated subquery, where the rows returned seem to depend upon the order
I specify my conditions.  I can't see why the order should be important
(except maybe for performance)

To demonstrate I created a toy data set and query (note, both are completely
contrived so neither really 'makes sense' beyond this context).  I present
first two queries which return different results, but differ *only in the
order I specify conditions*, along with the results of the queries.  (The
toy data set is at the very end of the email)

SELECT c.score FROM c 
WHERE c.score >= (SELECT MAX(score) AS score FROM c
			WHERE a.name='nugget'
			AND a.job='programmer'
			AND a.a_id=b.a_id AND c.b_id = b.b_id)
AND a.name='nugget'
AND a.job='programmer'
AND a.a_id=b.a_id
AND c.b_id = b.b_id;

 score 
-------
  2500
(1 row)

* NOTE, this query returns only one row.


SELECT c.score FROM c 
WHERE a.name='nugget' 
AND a.job='programmer'
AND a.a_id=b.a_id 
AND c.b_id = b.b_id 
AND c.score >= (SELECT MAX(score) AS score FROM c 
			WHERE a.name='nugget' 
			AND a.job='programmer' 
			AND a.a_id=b.a_id 
			AND c.b_id = b.b_id);

 score 
-------
   100
  2500
(2 rows)


* NOTE, now TWO rows are returned, even though all that has happened is I've
changed the order of the conditions.

What's going on here ? Why should the order of conditions be important ??
Any insight would be greatly appreciated.

Thanks,
terry

CREATE TABLE people (name text not null PRIMARY KEY, age int);
insert into people values ('nugget', 33);
insert into people values ('lisa', 32);
insert into people values ('larry', 28);

CREATE TABLE a (a_id INT NOT NULL PRIMARY KEY, job TEXT NOT NULL, name TEXT
NOT NULL, FOREIGN KEY (name) REFERENCES people(name));
INSERT INTO a VALUES (1, 'programmer', 'nugget');
INSERT INTO a VALUES (2, 'programmer', 'lisa');
INSERT INTO a VALUES (2, 'secretary', 'lisa');
INSERT INTO a VALUES (3, 'secretary', 'lisa');
INSERT INTO a VALUES (4, 'student', 'larry');
INSERT INTO a VALUES (5, 'programmer', 'nugget');

CREATE TABLE b (b_id INT NOT NULL PRIMARY KEY, a_id INT NOT NULL, FOREIGN
KEY (a_id) REFERENCES a(a_id));
INSERT INTO b (a_id, b_id) VALUES (1,1);
INSERT INTO b (a_id, b_id) VALUES (2,2);
INSERT INTO b (a_id, b_id) VALUES (3,3);
INSERT INTO b (a_id, b_id) VALUES (4,4);
INSERT INTO b (a_id, b_id) VALUES (5,5);

CREATE TABLE c (c_id INT NOT NULL PRIMARY KEY, b_id INT NOT NULL, score INT
NOT NULL, FOREIGN KEY (b_id) REFERENCES b(b_id));
INSERT INTO c (b_id, c_id, score) VALUES (1,1,100);
INSERT INTO c (b_id, c_id, score) VALUES (2,2,400);
INSERT INTO c (b_id, c_id, score) VALUES (3,3,900);
INSERT INTO c (b_id, c_id, score) VALUES (4,4,1600);
INSERT INTO c (b_id, c_id,score) VALUES (5,5,2500);


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Subject: Re: Weird (?) problem with order of conditions in SELECT
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