Upon generating an entity-relationship diagram of my database I observed several occurrence like this:
This diagram (in IDEF1X notation) shows the following:
iam_projects.db_idis a foreign key in
project_idfield), hence the connecting line.
iam_projects.db_idis not part of the primary key of
iam_project_rules(hence the dotted line style).
iam_projects.db_idmay appear zero or more times in
iam_project_rules(hence the solid circle on the right end of the connecting line).
Now, here's what my question is about. As I understand the notation, the diamond on the left end of the connecting line changes the left side from "one" to "zero or one". In words, that posits this claim to be true:
There may exist a row in
project_iddoes not appear in
iam_project_rules.project_id is a foreign key to
iam_projects, which it is, then that claim must be false.
Where is the error in my analysis or assumptions?