Re: protected ON DELETE CASCADE

Murray Hobbs wrote:
>
> here's my problem
>
> i have tables A, B, C, D
>
> A <- B
> A <- C <- D
>
> i want to maintain integrity so that if A is deleted from then so is
> anything referencing from B and C - no problem ON DELETE CASCADE
>
> but if there are any D's that point back to A (through composite key in
> C) i don't want the delete to go ahead - at all - i want an error
> message and condition

    So  B  and  C  reference  A  with  ON DELETE CASCADE, while D
    references C without it. The default behaviour of  a  foreign
    key  constraint  is  ON  DELETE  NO ACTION, which confusingly
    enough aborts the transaction (it's defined that way  in  the
    SQL  standard,  don't  ask  me why they called it NO ACTION).
    Thus a deletion from A will cascaded delete from C, but  then
    the  constraint  on  D  will  abort  the  transaction if this
    automatic delete from C would orphan a reference from D.


Jan

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