Convert ternary relationship to binary in E/R model


When I am studying the database lecture on E/R model, it illustrates how to convert ternary relationship to binary. One way is using weak entity relationship as follows (each relationship is M:N cardinality):

ternary relationship: ternary relationship

convert the upper relationship with weak relationship

convert the upper relationship with weak relationship

However, in another example: ternary relationship

it states in the lecture slide: "if each technician can be working on several projects and uses the same notebooks on each project, then we can decompose 3-ary relationship into binary relationships"as follows: enter image description here

which I could not understand. I still kinda confused about when we should use weak entity approach and when we could just simply convert it to binary relationships as the latter one. Thanks!


Answers:


Your second image illustrates a confusion between conceptual and physical data models, or a confusion between the ER and network data models. The physical implementation of the models in the first two images are the same, what differs is the interpretation of entities and relationships. The entity-relationship model supports ternary relationships, but doesn't support multiple identifying relationships for a single weak entity set. I would advise you to disregard the second image completely.

The third and fourth images illustrate a fourth normal form decomposition using ER notation. This isn't something you can do with any ternary relationship, but rather something you do when 2 or 3 independent relations have been incorrectly combined into one. For more information, I suggest you read up on Fourth Normal Form.