I'm stuck on a brain-teaser with CONSTRAINT: Imagine a table like "lineitems" in a bookstore - where you don't need an ISBN to be unique because a book will be in buying history more than once.
But you DO need to make sure that the ISBN number is ONLY matched to one book name - NOT to more than one book name. This is OK: isbn name 1234 Red Roses 1234 Red Roses This is OK: (two books can have the same name) isbn name 1234 Red Roses 5555 Red Roses This is NOT OK: (an isbn must be tied to one book only!) isbn name 1234 Red Roses 1234 Green Glasses I know it's tempting to say, "just link a separate table for the book and don't store the book name" but let's just pretend that's not an option - because I'm not actually dealing with books : I just made up this simplified version of something at work, where we can't change the table : both isbn and name MUST be in the table, and what I'm trying to do is put a CONSTRAINT on the table definition to protect against user error, by making sure that any entered isbn is only tied to one book-name in that table. Thoughts? ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
