----- Original Message ----- From: "A. Razzak Memon" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, April 09, 2002 12:13 PM Subject: Re: Please explain foreign keys, index - Razzak's Reply
> > At 04:00 PM 4/7/2002 -0400, Bernie Lis wrote: > > >what is diff between foreign key and foreign index? > >If I create a foreign index is that column indexed for > >look ups in that table. If I create a foreign key do I > >still have to build an index for look up in that table? > > Bernie, > > TTBOMK there is no such thing as Foreign Index. > > . An Index is automatically built when a foreign key constraint > is defined. > > . Key is a column or set of columns that identify a row. > > . All other columns should be dependent on a key. > > . A key is NOT an index. > > . A key is often indexed, but these terms are not interchangeable. > > . Primary Key is a column or set of columns that uniquely identify > a row. > > . Foreign Key is a column or set of columns that link to a Primary > Key. > > . A Foreign Key matches a defined Primary Key, i.e, the value in > a Foreign Key reference values in the Primary Key. > > . A Foreign Key can have duplicate values but cannot have NULLs. > > . The value in a Foreign Key must exist in a Primary Key and > Primary Key also prohibits NULLs. > > . A Foreign Key replaces a verify a value rule. > > . Primary and Foreign Keys must match in terms of the specified > number of columns. If you have a multi-column Primary Key, you > cannot have single column Foreign Keys reference it. If you have > a multi-column Foreign Key, it cannot reference a single Primary > Key. When a Primary Key is defined as more than one column, > those columns are treated as a whole. Primary and Foreign keys > must much exactly. > > Examples (CONCOMP Database): > > . EmpID is the Primary Key for the Employee table. > > . EmpID is a Foreign Key in TransMaster table. It relates to the > Primary Key in the Employee table. > > Hope that helps! > > Very Best Regards, > > Razzak. > > > > ================================================ > TO SEE MESSAGE POSTING GUIDELINES: > Send a plain text email to [EMAIL PROTECTED] > In the message body, put just two words: INTRO rbase-l > ================================================ > TO UNSUBSCRIBE: send a plain text email to [EMAIL PROTECTED] > In the message body, put just two words: UNSUBSCRIBE rbase-l > ================================================ > TO SEARCH ARCHIVES: > http://www.mail-archive.com/rbase-l%40sonetmail.com/ ================================================ TO SEE MESSAGE POSTING GUIDELINES: Send a plain text email to [EMAIL PROTECTED] In the message body, put just two words: INTRO rbase-l ================================================ TO UNSUBSCRIBE: send a plain text email to [EMAIL PROTECTED] In the message body, put just two words: UNSUBSCRIBE rbase-l ================================================ TO SEARCH ARCHIVES: http://www.mail-archive.com/rbase-l%40sonetmail.com/
