Message-ID: <4A16AD3B.5040407@cs.umu.se>
Date: Fri, 22 May 2009 15:48:43 +0200
From: Richard Ekblom <ens01rem@cs.umu.se>
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To: pgsql-sql@postgresql.org
Subject: Need help combining 2 tables together
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Hello

I have frequently encountered the need of combining two tables into one. 
First, please take a look at the following table setups...

CREATE TABLE topics (
   id SERIAL PRIMARY KEY,
   topic TEXT NOT NULL
);

CREATE TABLE messages (
   id SERIAL PRIMARY KEY,
   topic INTEGER REFERENCES topics(id),
   message TEXT NOT NULL
);

Example of a topics table:
ID    TOPIC
1     Are squares better then circles?
2     My favorite food

Example of a messages table:
ID    TOPIC    MESSAGE
1     2        I like lasagna!
2     2        Pizza is also a favorite
3     1        I like circles, they remind me of pizza

Notice that the number of topics may differ from the number of messages.

Now I want to combine these tables with a single SELECT to get...

Combined table:
ID   TOPIC                               MESSAGE
1    My favorite food                    I like lasagna!
2    My favorite food                    Pizza is also a favorite
3    Are squares better then circles?    I like circles, they remind me 
of pizza

I have seen different examples of this with something called JOIN but 
they always give me only two rows. How can I do this when the two tables 
may have different sizes to produce exactly the combined table above???


Some SQL for Postgres if you want to set up this example...

CREATE TABLE topics (id SERIAL PRIMARY KEY,topic TEXT NOT NULL);
CREATE TABLE messages (id SERIAL PRIMARY KEY,topic INTEGER REFERENCES 
topics(id),message TEXT NOT NULL);
INSERT INTO topics(topic) VALUES('Are squares better then circles?');
INSERT INTO topics(topic) VALUES('My favorite food');
INSERT INTO messages(topic,message) VALUES(2, 'I like lasagna!');
INSERT INTO messages(topic,message) VALUES(2, 'Pizza is also a favorite');
INSERT INTO messages(topic,message) VALUES(1, 'I like circles, they 
remind me of pizza');
SELECT * FROM topics;
SELECT * FROM messages;


Thanks in advance
/RE