Question

Many to many relationships should have a solid line, strong relationship.
In the given diagram, ther right side sample is surely incorrect, the relationship
line show be solid. However, what is the appropaite relationship for the left side diagram, should it be dashed (weak relationship) or solid?

Answer 1

Many to many relationships should have a solid line, strong relationship.

What notation are you using? Looks like a variant of IE (Information Engineering).

At any rate, I guess this depends on the notation/conventions being used and in this particular case the logical model seems consistent with the physical one, except for the requirement that a Project must be connected to at least one Employee, which cannot be easily¹ mapped to the physical database.

In the given diagram, ther right side sample is surely incorrect, the relationship line show be solid.

Not necessarily. There is nothing preventing you from migrating the parent PKs into an alternate key in the junction table and using a surrogate PK. This would make the relationships “non-identifying”, warranting the usage of dashed lines.

---

¹ It would require circular deferred FKs.