Between entity and entity there exists a relationship entity.
The cardinality between entities can be 1:1, 1:many, or many:many,
When the cardinality between entities A and B is many:many, operations are not possible.
There is no fixed anchor, so the primary key becomes ambiguous and duplicates can arise.
Therefore the relationship entity between the two entities
converts its role into an intersection entity (or action entity).
This changes the structure A(many) -- (in between) relationship entity -- entity(B) many
from A(many) : B entity, into
entity A(1_parent) : intersection entity(many_child) & intersection entity(many_child) : entity B(1_parent).
That is the new structure.
