A-Original-Client-UID: 188.129.105.225 splivalo
Message-ID: <4EE3DC17.3010804@megafon.hr>
Date: Sat, 10 Dec 2011 23:24:23 +0100
From: Mario Splivalo <mario.splivalo@megafon.hr>
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To: pgsql-sql@postgresql.org
Subject: Subselects to Joins? Or: how to design phone calls database
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I have a table called 'calls' which holds 'call detail records'. Let's
assume the table looks like this:

CREATE TABLE cdr (
	call_id serial,
	phone_number text
);

And I have a table with country call prefixes, that looks like this:

CREATE TABLE prefixes (
	prefix text,
	country text
);

And now some test data:

INSERT INTO prefixes VALUES ('1', 'USA');
INSERT INTO prefixes VALUES ('44', 'UK');
INSERT INTO prefixes VALUES ('385', 'Croatia');
INSERT INTO prefixes VALUES ('387', 'Bosnia');
INSERT INTO prefixes VALUES ('64', 'New Zeland');
INSERT INTO prefixes VALUES ('642', 'New Zeland Mobile');
INSERT INTO calls VALUES (1, '11952134451');
INSERT INTO calls VALUES (2, '448789921342');
INSERT INTO calls VALUES (3, '385914242232');
INSERT INTO calls VALUES (4, '385914242232');
INSERT INTO calls VALUES (5, '645122231241');
INSERT INTO calls VALUES (6, '444122523421');
INSERT INTO calls VALUES (7, '64212125452');
INSERT INTO calls VALUES (8, '1837371211');
INSERT INTO calls VALUES (9, '11952134451');
INSERT INTO calls VALUES (10, '448789921342');
INSERT INTO calls VALUES (11, '385914242232');
INSERT INTO calls VALUES (12, '385914242232');
INSERT INTO calls VALUES (13, '645122231241');
INSERT INTO calls VALUES (14, '4441232523421');
INSERT INTO calls VALUES (15, '64112125452');
INSERT INTO calls VALUES (16, '1837371211');


Now, if I want to have a 'join' between those two tables, here is what I
am doing right now:

SELECT
	call_id,
	phone_number,
	(SELECT
		country
	FROM
		prefixes
	WHERE	
		calls.phone_number LIKE prefix || '%'
	ORDER BY
		length(prefix) DESC LIMIT 1
	) AS country
FROM calls;


Is there a way I could use join here? I can do something like:

SELECT ... FROM calls JOIN prefixes ON calls.phone_number LIKE prefix || '%'

but I'd get duplicate rows there (for instance, for New Zeland calls,
from my test data).

Or should I add 'prefix' field to the calls table, and then do a inner
join with prefixes table?

	Mario