RE: [PHP-DB] foreign key problem
Yes sir, you are correct. Thank you for correcting me. In practice, a NULL in a foreign key almost always means an orphan record. In most applications the designer will want to use the NOT NULL constraint. Bob Hall >A foreign key value can be null, if it suits the data application - or that >is what I've always been taught. > >Here is a text book definition of Referential Integrity which was spoon fed >to me by the Open University when studying their RDBMS course: > >"Referential Integrity - If a relation (table), R2 has a foreign key, F, >that references the primary key, P, in another relation (table), R1, then >every R2.F entry must either be a value equal to an R1.P primary key value >or be null" > >Regards, >Chris > > > -Original Message- > > From: Bob Hall [mailto:[EMAIL PROTECTED]] > > Sent: 31 January 2001 01:53 > > To: [EMAIL PROTECTED] > > Subject: Re: [PHP-DB] foreign key problem > > > > > > >hello all! > > >i have a little problem ,hope someone can help me out > > >the problem is : > > >let i have two tables T1 and T2 ,now T1 has following > > >fields > > >cntryid > > >cntryname > > >cntrycode etc. > > >now cntryid is the primary key now i want to make the > > >cntryid a foreign key in table T2 .so how can i do it > > >plz help me . > > >msjamal > > > > A column is a foreign key because it contains only values found in > > the referenced key, and no NULLs. Design your database so that the > > foreign key column contains only values from cntryid. > > > > If you want to know how to declare a referenced key/foreign key > > relationship, you will have to specify what RDBMS you are using. > > > > Bob Hall > > > > Know thyself? Absurd direction! > > Bubbles bear no introspection. -Khushhal Khan Khatak > > > > -- > > PHP Database Mailing List (http://www.php.net/) > > To unsubscribe, e-mail: [EMAIL PROTECTED] > > For additional commands, e-mail: [EMAIL PROTECTED] > > To contact the list administrators, e-mail: [EMAIL PROTECTED] > > >-- >PHP Database Mailing List (http://www.php.net/) >To unsubscribe, e-mail: [EMAIL PROTECTED] >For additional commands, e-mail: [EMAIL PROTECTED] >To contact the list administrators, e-mail: [EMAIL PROTECTED] Know thyself? Absurd direction! Bubbles bear no introspection. -Khushhal Khan Khatak -- PHP Database Mailing List (http://www.php.net/) To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] To contact the list administrators, e-mail: [EMAIL PROTECTED]
RE: [PHP-DB] foreign key problem
A foreign key value can be null, if it suits the data application - or that is what I've always been taught. Here is a text book definition of Referential Integrity which was spoon fed to me by the Open University when studying their RDBMS course: "Referential Integrity - If a relation (table), R2 has a foreign key, F, that references the primary key, P, in another relation (table), R1, then every R2.F entry must either be a value equal to an R1.P primary key value or be null" Regards, Chris > -Original Message- > From: Bob Hall [mailto:[EMAIL PROTECTED]] > Sent: 31 January 2001 01:53 > To: [EMAIL PROTECTED] > Subject: Re: [PHP-DB] foreign key problem > > > >hello all! > >i have a little problem ,hope someone can help me out > >the problem is : > >let i have two tables T1 and T2 ,now T1 has following > >fields > >cntryid > >cntryname > >cntrycode etc. > >now cntryid is the primary key now i want to make the > >cntryid a foreign key in table T2 .so how can i do it > >plz help me . > >msjamal > > A column is a foreign key because it contains only values found in > the referenced key, and no NULLs. Design your database so that the > foreign key column contains only values from cntryid. > > If you want to know how to declare a referenced key/foreign key > relationship, you will have to specify what RDBMS you are using. > > Bob Hall > > Know thyself? Absurd direction! > Bubbles bear no introspection. -Khushhal Khan Khatak > > -- > PHP Database Mailing List (http://www.php.net/) > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > To contact the list administrators, e-mail: [EMAIL PROTECTED] -- PHP Database Mailing List (http://www.php.net/) To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] To contact the list administrators, e-mail: [EMAIL PROTECTED]
Re: [PHP-DB] foreign key problem
>hello all! >i have a little problem ,hope someone can help me out >the problem is : >let i have two tables T1 and T2 ,now T1 has following >fields >cntryid >cntryname >cntrycode etc. >now cntryid is the primary key now i want to make the >cntryid a foreign key in table T2 .so how can i do it >plz help me . >msjamal A column is a foreign key because it contains only values found in the referenced key, and no NULLs. Design your database so that the foreign key column contains only values from cntryid. If you want to know how to declare a referenced key/foreign key relationship, you will have to specify what RDBMS you are using. Bob Hall Know thyself? Absurd direction! Bubbles bear no introspection. -Khushhal Khan Khatak -- PHP Database Mailing List (http://www.php.net/) To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] To contact the list administrators, e-mail: [EMAIL PROTECTED]
[PHP-DB] foreign key problem
hello all! i have a little problem ,hope someone can help me out the problem is : let i have two tables T1 and T2 ,now T1 has following fields cntryid cntryname cntrycode etc. now cntryid is the primary key now i want to make the cntryid a foreign key in table T2 .so how can i do it plz help me . msjamal __ Get personalized email addresses from Yahoo! Mail - only $35 a year! http://personal.mail.yahoo.com/ -- PHP Database Mailing List (http://www.php.net/) To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] To contact the list administrators, e-mail: [EMAIL PROTECTED]
