F
Fred
In our case, duplicates of the two linking fields in the junction table are
common and a part of the plan. One example is where the "type" of
relationship is a field in the junction table, and two entities can have
several types of relationships. Another is BOM / manufacturing DB's (M-M
between assemblies and the parts used to build them) when a junction table
record is an "instance of use" where there is other info related to that
intance of use. ( e.g. an IC might be used twice on a circuit board, once
as "IC1" and once as "IC2".)
So I never thought of situations where the junction table needs to enforce
rules as you describe.
common and a part of the plan. One example is where the "type" of
relationship is a field in the junction table, and two entities can have
several types of relationships. Another is BOM / manufacturing DB's (M-M
between assemblies and the parts used to build them) when a junction table
record is an "instance of use" where there is other info related to that
intance of use. ( e.g. an IC might be used twice on a circuit board, once
as "IC1" and once as "IC2".)
So I never thought of situations where the junction table needs to enforce
rules as you describe.