Two joins on same foreign key

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

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

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

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

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

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

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

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