Re: Create a foreign key from a field that is part of a mulitple primary key

"Mark C. Stock" <mcstockX_at_Xenquery .com> wrote:

>
>"naud" <naudjf_at_yahoo.fr> wrote in message
>news:1e1a7efc.0409020717.48e48228_at_posting.google.com...
>| Hi,
>|
>| i had a table named T_A, with a double primary key : (A,B)
>|
>| Then i created a table named T_B with a field : C.
>|
>| I want this field T_B.C to take only values that already exist in
>| field T_A.A.
>|
>| So i tried to create a foreign key on my field T_B.C, pointing on the
>| field T_A.A : an error message appeared "not possible to create a
>| foreign key on a field that is not a primary key".
>|
>| How can i solve this....? If U have any idea, please mail me !!
>|
>| THANX very much.
>
>
>whenever you have a multi-part primary key, as in T_A(A,B), you have either
>a dependent table or an intersection table -- that implies that part of the
>PK is actually an FK to another table.
>
>so in your case, column A looks like it should be an FK to the PK of another
>table 'T_X' (which appears to be missing)
>
>once that table (T_X) is created with the appropriate PK (and TA(A) is
>declared as an FK to it) then you can create an FK on T_B(C) referencing the
>correctly modelled PK of table 'T_X'
>
>++ mcs
>

I do not follow your
"whenever you have a multi-part primary key, as in T_A(A,B), you have either a dependent table or an intersection table -- that implies that part of the PK is actually an FK to another table."

We have multi-field ( compound) Primary Keys in several tables since the only way to get a unique value for those tables is to use more than one field..It has nothing to do with dependent or intersection tables... Received on Thu Sep 02 2004 - 11:35:55 CDT