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RE: Number of joins in the query

From: Naveen Nahata <naveen_nahata_at_mindtree.com>
Date: Thu, 27 Feb 2003 22:38:58 -0800
Message-ID: <F001.0055C8C7.20030227223858@fatcity.com> 





Since the primary key/foregin key can be composite, there can be more than
n-1 join conditions, but there will be only n-1 joins.



e.g. A's primary key is col1 and col2 referred by B's col1 and col2



in this case you will only have 1 join, between A and B, but 2 join
conditions 



A.col1 = B.col1 and A.col2 = B.col2



but in no case you should join A to B, B to C and then A to C again. If you
need to do that, then go one step back and look at the design. 



Regards


Naveen



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


Sent: Thursday, February 27, 2003 11:36 PM
To: Multiple recipients of list ORACLE-L




If you are joining 'n' number of tables, you should have a minimum of 'n-1'
joining conditions, otherwise it might result in cartesian product.



Sunil Nookala


Dell Corp.





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


Sent: Thursday, February 27, 2003 10:29 AM
To: Multiple recipients of list ORACLE-L




Two!



PK on stn_rstcn_to_frm?



T'would be clearer if you presented your problem as tables a, b, c, rather
than getting us to struggle through all these stns...



peter




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


[mailto:Ranganath.Krishnaswamy_at_blr.hpsglobal.com]
Sent: 27 February 2003 14:59


To: Multiple recipients of list ORACLE-L




Dear List,



        I have a basic doubt about the number of joins I should have.  Say,
I have three tables by name station, station_restriction and
stn_rstcn_to_frm with the following structure:


Station                Station_restriction             stn_rstcn_to_frm
----------               ----------------------------
-------------------------
stn_key(PK)          stn_rstcn_key(PK)            stn_rstcn_key(FK)
station_code         stn_key(FK)                      stn_key(FK)
station_name        restricted_position              country

	If I have to select data from all the three tables should I have two


joins or three joins?



            If I have two joins, I would have 



        Station.stn_key=Station_restriction.stn_key and
Station_restriction.stn_rstcn_key = stn_rstcn_to_frm.stn_rstcn_key.



        If I have three joins I would have 



           Station.stn_key=Station_restriction.stn_key and
Station_restriction.stn_rstcn_key = stn_rstcn_to_frm.stn_rstcn_key 


           and Station.stn_key = stn_rstcn_to_frm.stn_key



        Can anybody let me know as to how many joins should I have so that I
don't get any cartesian product in the result set?



        I am sorry if the question sounds trivial to someone but I got this
basic doubt while writing a complex query for a multi-table join.



        Any help in this regard is very much appreciated.



Thanks and Regards,



Ranganath

        


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  INET: Sunil_Nookala_at_Dell.com


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Author: Naveen Nahata


  INET: naveen_nahata_at_mindtree.com


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Received on Fri Feb 28 2003 - 00:38:58 CST












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