Between two entities is a 1 to many relationship but this relationship has an attribute of its own.
When converting these entities to a relational schema what will happen to this attribute? Does it get joined into the entity on the many side of the relationship like a foreign key does? I don't think it should be another separate relation because that is like saying it's an associative entity which it's not.
Any help is greatly appreciated.
In ER as described by Chen, each entity relation and each relationship relation would map to a separate table (except weak entities/identifying relationships). However, one-to-one and one-to-many relationships commonly get denormalized to reduce the number of tables. In this case, attributes of the relationship become attributes of the determining entity.
Associative entities occur when a relationship is the subject of a relationship. Recording a relationship in a separate table doesn't make it an associative entity. Nor do attributes make a relationship into an entity. Attributes on relationships are quite normal in the ER model.
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Is it possible to model a recursive relationship (such as seen in the photo) using the relational model? I find it hard to believe that the relation user would receive a foreign key from itself to signify the 1:n relationship it has with itself in the relationship "is_referred_by".
An entity in an EER Diagram with a recursive relationship
Let's assume we have 2 ERD entities connected with 1 relationship. Visual Paradigm allows to mark such a relationship as "Subtype" and "Identifying" at the same time. And I just fail to imagine any case where such a combination could make sense.
Is that actually legal? If yes I would appreciate some example and a procedure of translating it to the relational model (what are the changes in comparison to only-"Subtype" or only-"Identifying" case?).
What Visual Paradigm calls an ERD is in fact a table diagram. Diagrams that don't use shapes for relationships don't support attributes and relationships on relationships, nor ternary and higher order relationships. If a diagramming notation doesn't support all the concepts of the Entity-Relationship model, we can't call its diagrams Entity-Relationship diagrams.
From an ER point of view, all subtype relationships are identifying relationships, since the parent entity set's identity is a component of the child entity set's identity. However, all identifying relationships aren't subtype relationships. The difference between a subtype and a weak entity set is that the latter uses an additional weak key component to distinguish multiple children of a parent. Subtypes depend only on the supertype for identity and so each instance of the supertype can have only instance per subtype.
For example, an entity set Person (identified by person_id) may have a subtype Employee (which is also identified by person_id). Compare this with an Invoice (identified by invoice_id) and LineItem (identified by invoice_id and line_number).
The ERD (conceptual model) translates into the following tables (physical model):
Obviously a weak entity must be involved (as the child) in one or more identifying relationships, otherwise it would not be weak.
But can it, at the same time, also be the child in a non-identifying relationship?
Yes, weak entity sets can participate in non-identifying relationships with other entities. By "child", I suppose you mean relationships in which the weak entity set is on the determining (many) side of the relationship. There is no restriction on the kinds of relationships in which weak entity sets may participate.
However, watch out for multiple identifying relationships. While a weak entity set can easily be the descendant of a chain of identifying relationships, multiple independent identifying relationships on the same weak entity set are unusual, and challenges the very notion of an entity set. See my answer to this question.
Note that your question title mentions relationships between tables, which confuses conceptual (ER) and physical (tabular) models. Entity relations are represented by tables, but so are relationship relations in general (only one-to-one and one-to-many relationships can be denormalized into the entity relation of a/the determining entity set, many-to-many and ternary and higher relationships still require their own tables).
The ER diagram for a database is given below:
Now, what's the relationship between planes and flights entities?
I'd say one to many, but that'd be wrong because while one plane can have more than one flights, many flights can't have one (single) plane simultaneously.
So, what exactly is the relationship?
I'm new to databases. Please tell me if I'm wrong.
First, your diagram isn't an ER diagram, it's a table diagram. ER diagrams must be able to represent the ER model, which supports ternary and higher relationships, weak entity sets and identifying relationships, and other concepts for which table diagrams don't have notation. Proper ER diagram notation is generally referred to as Chen's notation.
Now, ER relationships are easily identified by looking for entity domains (columns that identify entity sets) that appear in the same table. These are usually indicated by PK and/or FK, but they need not be.
When you have a binary relationship (two entity domains in the same table), look at the unique constraints on these domains:
One-to-one relationships require two separate unique constraints, one for each domain.
One-to-many relationships require a unique constraint on the domain on the many side.
Many-to-many relationships require a composite unique constraint on both domains together.
One-to-many relationships can be denormalized into the entity relation of the entity on the many side, since the unique constraint required for the relationship matches the PK for the entity relation. One-to-one relationships can be denormalized into either entity relation. A many-to-many relationship requires a composite key, and must be represented as a separate relation since the composite key doesn't match either entity relation's PK.
In your example, (flight_num, planeID) represents the relationship, and since only flight_num is uniquely constrained (due to being the PK of the flights relation), this is a many-to-one relationship: each flight is associated with exactly one plane, while each plane can be involved in many flights.
Here's a visual reference in which unique constraints are indicated with underlining:
Many people still use terminology and concepts from the old network data model, such as conflating relationships with FK constraints and entity sets with tables (which is why table diagrams are often mistakenly called ERDs). I highly recommend reading Codd's paper "A Relational Model of Data for Large Shared Data Banks" and Chen's paper "The Entity-Relationship Model - Toward a Unified View of Data".
As you said the relation is One-To-Many because if it is a Many-To-Many relation it must have a junction table between these 2 tables (Tbl_Plane_Flight) , another thing is that Plane_Id is referenced in Flights table.
But there must be a validation rule (or a constraint in flights table) for this relation, that a plane cannot have many flight as the same time
I have the following as my ER Diagram. I am still learning DBMS, and trying to translate this ER Diagram to a Relational Schema. I know that each entity in the ER Diagram will have a separate table. However, I am not sure what to do about the relationship for this particular ER Diagram. We were told that each relationship between the entities will have a table too. Therefore, do I need to make separate for relationships in this ER Diagram as well? But, there are no attributes of the relationship. Also, I am having a confusion in what sort of relationship is this exactly? Is this one to many?
I am attaching the link to the picture of the ER Diagram. Please guide me in the right direction. Thanks!
Your diagram isn't an ER diagram in the original sense of that term. In the entity-relationship model, relationships are associations among entity sets and were meant to be implemented as tables. For example, your AUTHOR_BOOK, CAST and PURCHASE tables are relationship tables that associate two entity sets each (keep in mind that relationships aren't limited to just two entity sets). Note how the relationships are represented using the keys of the entity sets e.g. (actorID, inventID). The same pattern can be found in some of your other tables, i.e. (inventID, publisher), (inventID, director), (inventoryID, genre), (inventoryID, supplier), (receiptID, inventID) and (receiptID, customerID). These are your relationships - not the crow's foot lines which are just foreign key constraints. In Chen's original notation, the relationships would be indicated using diamond shapes between and connected to the two entity types. Also, Chen would've made a separate relationship table (aka junction table) for each of these relationships.
Your table diagram shows 14 tables. Following Chen's method, there would be 19 tables:
Your title references a relational schema. Note that relational schemas are not restricted to the entity-relationship model, but can represent any set of normalized tables (1NF or higher). The number of tables would partially depend on the level of normalization.
But, there are no attributes of the relationship.
This is not correct. Your Purchase relationship shows two attributes - quantity and amountPaid. Note that an attribute is a mapping from an entity or relationship set to a value set. Thus, I'm not counting the entity keys as attributes of the relationship. I also modeled Book's pubYear as an attribute of the relationship between Book and Publisher.
In practice, I would probably denormalize relations with the same determinant, which give a physical schema similar to your original diagram, though implementing every relationship table separately does have some advantage in easing schema changes when relationship cardinalities change.