Thread: Two joins on same foreign key

Two joins on same foreign key

From

Anony Mous

Date:

30 January 2004, 21:09:51

Hi,

I’m fairly new to this database, and have read much discussion on sub-queries. I’ve seen that they can be great for some queries, and downright slow for others. I have a table with two foreign keys referencing another table, like:

Table #1

employee_id (pk)

employee_name

Table #2

teamleader_employee_id

backup_employee_id

both fields in table 2 need to do a lookup in table 1 to get the name of the actual employee. Do I need to use nested queries to accomplish this? Any help is greatly appreciated!

-AM

Re: Two joins on same foreign key

From

Gregory Wood

Date:

30 January 2004, 21:43:24

> I’m fairly new to this database, and have read much discussion on
> sub-queries.  I’ve seen that they can be great for some queries, and
> downright slow for others.  I have a table with two foreign keys
> referencing another table, like:
>
> Table #1
>
> employee_id (pk)
>
> employee_name
>
> Table #2
>
> teamleader_employee_id
>
> backup_employee_id
>
> both fields in table 2 need to do a lookup in table 1 to get the name of
> the actual employee.  Do I need to use nested queries to accomplish
> this?  Any help is greatly appreciated!

Just do two joins against the first table:

SELECT *
FROM table2
JOIN table1 tl ON (teamleader_employee_id=tl.employee_id)
JOIN table1 b  ON (backup_employee_id=b.employee_id);

Greg

Re: Two joins on same foreign key

From

Gregory Wood

Date:

31 January 2004, 02:29:57

Anony Mous wrote:
>>I'm fairly new to this database, and have read much discussion on
>>sub-queries.  I've seen that they can be great for some queries, and
>>downright slow for others.  I have a table with two foreign keys
>>referencing another table, like:
>>
>>Table #1
>>
>>employee_id (pk)
>>
>>employee_name
>>
>>Table #2
>>
>>teamleader_employee_id
>>
>>backup_employee_id
>>
>>both fields in table 2 need to do a lookup in table 1 to get the name of
>>the actual employee.  Do I need to use nested queries to accomplish
>>this?  Any help is greatly appreciated!
>
>
> Just do two joins against the first table:
>
> SELECT *
> FROM table2
> JOIN table1 tl ON (teamleader_employee_id=tl.employee_id)
> JOIN table1 b  ON (backup_employee_id=b.employee_id);
>
 > Funny, I still can't get it to work.  Postgresql complains there are
 > two joins on a single table and won't do it!  Are you sure that this
 > can be done?

Yes, I do it myself... I've even joined a table to itself. What error
message do you get? Make sure that you assign an alias like my example.

test=# CREATE TABLE table1 (employee_id SERIAL PRIMARY KEY, name VARCHAR);
CREATE TABLE
test=# INSERT INTO table1 (name) VALUES ('Employee 1');
INSERT 104693 1
test=# INSERT INTO table1 (name) VALUES ('Employee 2');
INSERT 104694 1
test=# CREATE TABLE table2 (id SERIAL PRIMARY KEY,
teamleader_employee_id INTEGER REFERENCES table1 (employee_id),
backup_employee_id INTEGER REFERENCES table1 (employee_id));
CREATE TABLE
test=# INSERT INTO table2 (teamleader_employee_id, backup_employee_id)
VALUES (1, 2);
INSERT 104717 1
test=# SELECT *
test-# FROM table2
test-# JOIN table1 tl ON (teamleader_employee_id=tl.employee_id)
test-# JOIN table1 b  ON (backup_employee_id=b.employee_id);
  id | teamleader_employee_id | backup_employee_id | employee_id |
name    | employee_id |    name
----+------------------------+--------------------+-------------+------------+-------------+------------
   1 |                      1 |                  2 |           1 |
Employee 1 |           2 | Employee 2
(1 row)

Greg

Re: Two joins on same foreign key

From

Michael Glaesemann

Date:

31 January 2004, 05:20:35

On Jan 31, 2004, at 7:03 AM, Anony Mous wrote:
> Table #1
> employee_id (pk)
> employee_name 
>
> Table #2
> teamleader_employee_id
> backup_employee_id
>
>  both fields in table 2 need to do a lookup in table 1 to get the name
> of the actual employee.  Do I need to use nested queries to accomplish
> this?  Any help is greatly appreciated!

I think you could handle this by calling table1 twice, but with
different aliases, like so

SELECT leader.employee_name, backup.employee_name
FROM table1 leader, table1 backup, table2 t2
WHERE
    leader.employee_id = t2.teamleader_employee_id AND
    backup.employee_id = t2.backup_employee_id

Does that work for you?

Michael Glaesemann
grzm myrealbox com

Re: Two joins on same foreign key

From

Anony Mous

Date:

31 January 2004, 05:41:38

Actually, given that there may not always be a backup_employee field (can be
null) I think I'm forced to use an outer join to return all team leader
records regardless if a matching backup_employee record exists.  I'll test
yours out and see.

Thank you!



-----Original Message-----
From: Michael Glaesemann [mailto:grzm@myrealbox.com]
Sent: January 30, 2004 11:21 PM
To: Anony Mous
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Two joins on same foreign key


On Jan 31, 2004, at 7:03 AM, Anony Mous wrote:
> Table #1
> employee_id (pk)
> employee_name 
>
> Table #2
> teamleader_employee_id
> backup_employee_id
>
>  both fields in table 2 need to do a lookup in table 1 to get the name
> of the actual employee.  Do I need to use nested queries to accomplish
> this?  Any help is greatly appreciated!

I think you could handle this by calling table1 twice, but with
different aliases, like so

SELECT leader.employee_name, backup.employee_name
FROM table1 leader, table1 backup, table2 t2
WHERE
    leader.employee_id = t2.teamleader_employee_id AND
    backup.employee_id = t2.backup_employee_id

Does that work for you?

Michael Glaesemann
grzm myrealbox com

Re: Two joins on same foreign key

From

Michael Glaesemann

Date:

31 January 2004, 09:49:20

On Jan 31, 2004, at 3:35 PM, Anony Mous wrote:

> Actually, given that there may not always be a backup_employee field
> (can be
> null) I think I'm forced to use an outer join to return all team leader
> records regardless if a matching backup_employee record exists.

yup. Actually, I hadn't seen the other responses when I first posted.
You've got some options, it looks like. :)
Michael Glaesemann
grzm myrealbox com

Re: Two joins on same foreign key

From

elein

Date:

31 January 2004, 18:11:05

PostgreSQL General Bits Issue #56 has an article on Join Basics
which also has an example of multiple table joins.

http://cookie.varlena.com/varlena/GeneralBits/56.php

--elein
elein@varlena.com

On Fri, Jan 30, 2004 at 03:03:35PM -0700, Anony Mous wrote:
> Hi,
>
>
>
> I??m fairly new to this database, and have read much discussion on
> sub-queries.  I??ve seen that they can be great for some queries, and downright
> slow for others.  I have a table with two foreign keys referencing another
> table, like:
>
>
>
> Table #1
>
> employee_id (pk)
>
> employee_name
>
>
>
> Table #2
>
> teamleader_employee_id
>
> backup_employee_id
>
>
>
> both fields in table 2 need to do a lookup in table 1 to get the name of the
> actual employee.  Do I need to use nested queries to accomplish this?  Any help
> is greatly appreciated!
>
>
>
> -AM
>

Re: Two joins on same foreign key

From

Anony Mous

Date:

31 January 2004, 18:22:54

I must say, I'm really appreciative with the responses from this list.
Thanks to all!




-----Original Message-----
From: elein [mailto:elein@varlena.com]
Sent: January 31, 2004 12:10 PM
To: Anony Mous
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Two joins on same foreign key

PostgreSQL General Bits Issue #56 has an article on Join Basics
which also has an example of multiple table joins.

http://cookie.varlena.com/varlena/GeneralBits/56.php

--elein
elein@varlena.com

On Fri, Jan 30, 2004 at 03:03:35PM -0700, Anony Mous wrote:
> Hi,
>
>
>
> I??m fairly new to this database, and have read much discussion on
> sub-queries.  I??ve seen that they can be great for some queries, and
downright
> slow for others.  I have a table with two foreign keys referencing another
> table, like:
>
>
>
> Table #1
>
> employee_id (pk)
>
> employee_name
>
>
>
> Table #2
>
> teamleader_employee_id
>
> backup_employee_id
>
>
>
> both fields in table 2 need to do a lookup in table 1 to get the name of
the
> actual employee.  Do I need to use nested queries to accomplish this?  Any
help
> is greatly appreciated!
>
>
>
> -AM
>

Re: Two joins on same foreign key

From

elein

Date:

31 January 2004, 19:35:36

Ooops.  I've sent the wrong URL for my own darn site.
This is the correct URL.

http://www.varlena.com/GeneralBits/56.php

Sorry for the confusion.

--elein

On Sat, Jan 31, 2004 at 11:09:37AM -0800, elein wrote:
> PostgreSQL General Bits Issue #56 has an article on Join Basics
> which also has an example of multiple table joins.
>
> http://cookie.varlena.com/varlena/GeneralBits/56.php
>
> --elein
> elein@varlena.com
>
> On Fri, Jan 30, 2004 at 03:03:35PM -0700, Anony Mous wrote:
> > Hi,
> >
> >
> >
> > I??m fairly new to this database, and have read much discussion on
> > sub-queries.  I??ve seen that they can be great for some queries, and downright
> > slow for others.  I have a table with two foreign keys referencing another
> > table, like:
> >
> >
> >
> > Table #1
> >
> > employee_id (pk)
> >
> > employee_name
> >
> >
> >
> > Table #2
> >
> > teamleader_employee_id
> >
> > backup_employee_id
> >
> >
> >
> > both fields in table 2 need to do a lookup in table 1 to get the name of the
> > actual employee.  Do I need to use nested queries to accomplish this?  Any help
> > is greatly appreciated!
> >
> >
> >
> > -AM
> >
>
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