Primary Keys DO NOT permit NULLS, however Foreign Keys, by default, DO.
Tony
[EMAIL PROTECTED]
04/09/2002 01:53 PM
Please respond to rbase-l
To: <[EMAIL PROTECTED]>
cc: (bcc: Anthony Schmidt/BayShore/SGU_LN)
Subject: Re: Please explain foreign keys, index - Razzak's Reply -
correction
regarding NULL status of Foreign Keys
I would like to see that produced on virgin tables. Primary Key does not
allow nulls. Data row in FK table cannot be created unless data exists in
PK table..
----- Original Message -----
From: "Anthony Schmidt" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, April 09, 2002 12:28 PM
Subject: Re: Please explain foreign keys, index - Razzak's Reply -
correction regarding NULL status of Foreign Keys
> Correction -
>
> A Foreign Key can be NULL.
>
> A trap for the unwary is that unless you select the NOT NULL constraint
> when creating a Foreign Key, your Foreign Key will permit NULL values.
>
> This IS NOT an RBase bug because it conforms to the SQL standard.
>
> You can change the NOT NULL constraint for the Foreign Key after the
fact.
>
>
>
> Tony
>
> Anthony J. Schmidt, JD
> President
> The Computery Ltd.
> One East Main Street
> Bay Shore, NY 11706
>
> 631-665-8100 Voice
> 631-969-5988 FAX
>
> http://www.computeryltd.com
>
>
>
>
>
> [EMAIL PROTECTED]
> 04/09/2002 12:13 PM
> Please respond to rbase-l
>
>
> To: [EMAIL PROTECTED]
> cc: (bcc: Anthony Schmidt/BayShore/SGU_LN)
> 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/
>
================================================
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/
