a simple table:
ID, NAME, POST_ID, GROUP_ID either POST_ID or GROUP_ID must be set, but never both of them, NEITHER none of them. So,
there are valid cases:
ID, NAME, POST_ID, GROUP_ID x, y, 1, NULL x, y, NULL, 4 and NOT VALID cases:
ID, NAME, POST_ID, GROUP_ID x, y, NULL, NULL x, y, 4, 4 is it possible to set such complicated restriction rule?
I don't think it's possible. I would catch such cases in the front-end where the user enters the data. Also, you can perhaps replace both fields with two other fields:
A category field of type enum with possible values 'post' or 'group' A reference field containing the ID.
That way you can easily demand both to be non-empty and you can only reference a post ID or a group ID.
