But if referential integrity is taken how null are allowed as b is key I think min will be one
By allowing all duplicates in R of b
A similar question but "self-join" makes this highly complex:
For referential integrity to hold in a relational database, any field in a table that is declared a foreign key can contain either a null value, or only values from a parent table's primary key or a candidate key. In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.