In Entity Relationship Diagram defines the relationship between entities and their attributes. In example, an ERD contains many-to-many relationships and do not include foreign keys. They areimplied. In an ERD, we are simply visualising the entities, their attributes, and the relation between them. we dont care about where foreign keys go or how to implement that many-to-many relationship. (but we do underline primary keys!)

In a Relational Model we are referring to an implementation of our model. At this point we should be clear about the formats of our attributes, the foreign keys we use, and the linking tables we might need to stitch entities together. A relational model is basically a model of a possible database implementation.
In short, an ERD is an abstract concept of our database, it speaks in entities and attributes, an entity model. A relational model defines formats and relations in a way a database could understand, a data model.

hope this info will be usefull for you!! Enjoy :) 

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