Usually we don’t distinguish between many2one and one2many, since it’s the same just viewed from the other entity.
There is one more class though, which is one2one. That is, the entities have a direct relationship. Sometimes this also includes the case where you have zero or one, i.e. the relation is optional on one side. This can be accomplished with an FK plus unique constraint or by merging the tables.
Usually we don’t distinguish between many2one and one2many, since it’s the same just viewed from the other entity.
There is one more class though, which is one2one. That is, the entities have a direct relationship. Sometimes this also includes the case where you have zero or one, i.e. the relation is optional on one side. This can be accomplished with an FK plus unique constraint or by merging the tables.