Re: Multi column check constraint

Jeff> Can a check constraint be written such that if item_link_no is null then Jeff> voucher_id must be not null and vice versa?

It depends on the database; not all of them support table constraints (i.e. constraints that involve more than one column). If the do, following is what it would look like. This is Oracle syntax, but I don' tthink it's far off the standard:

alter table foobar add constraint exactly_one_null check (

(item_link_no is not null and voucher_id is not null) or
(item_link_no is null and voucher_id is not null) or
(item_link_no is not null and voucher_id is null));

A more elegant way is to translate the columns to 0/1 integers; using oracle's decode (and others db's have similar functions):

 check( decode(item_link_no,null,0,1)+decode(voucher,null,0,1) > 0);

                                                                      Philip

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