Обсуждение: Need help combining 2 tables together

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

On Friday 22 May 2009 6:48:43 am Richard Ekblom wrote:
> 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

test=# SELECT m.id,t.topic,m.message from topics as t,messages as m where 
m.topic=t.id order by m.id;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
 


-- 
Adrian Klaver
aklaver@comcast.net

----- Original Message -----

From: James Kitambara

To: Richard Ekblom

Cc: pgsql-sql@postgresql.org

Sent: Friday, May 22, 2009 3:47 PM

Subject: Re: [SQL] Need help combining 2 tables together

Dear Richard Ekblom, I think Mr. Adrian Klaver gave you the solution. Mine is the similar solution SELECT message.id,topic.topic,message.message FROM topics, messages WHERE message.topic=topic.id order by message.id;   After executing this query you will get the following: 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   Best Regards,   Muhoji James Kitambara Database Administrator, B.Sc. With Computer Science and Statistics (Hons), National Bureau of Statistics, P.O. Box 796, Tel : +255 22 2122722/3    Fax: +255 22 2130852, Mobile : +255 71 3307632, Dar es Salaam, Tanzania.   -----------------------------------------ORGINAL MESSAGE-------------------------------- On Friday 22 May 2009 6:48:43 am Richard Ekblom wrote: > 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??? > >