I have the following ER model:
User can have multiple Accounts and every Account itself can have many Transactions. But in my model I have the relationship between two weak entities - Account and Transaction, and I cannot figure out how I should create my tables, specially Transaction table. What keys I should define and how I should relate them in the Transaction table? Thanks in advance.
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i am having a hard time creating an ERD for my table relationship. I have 4 tables: film, ticket_type, studio, and schedule. The schedule table is a relationship table that contain the primary key from the other three tables as foreign key. The question is how can i picture it in ERD? Something like many-to-many relationship but with 3 table, is it possible to do it like this? The database works fine when i try to create it so i think there's no problem in my concept. Thanks in advance.
Edit: forgot to add the ticket_type table is for pricing and type like: 2d,3d,or 4d, i create it like this to avoid redundancy.
One more question, can i add another field to a relationship table? If I remember correctly it should be fine, but just to make sure.
If schedule is a relationship, it would be represented as follows on an entity-relationship diagram:
Relationships are identified by the keys of the related entities. A table diagram makes this more visible:
However, if schedule is an entity set with relationships to the other 3 entity sets, it would be represented as follows on an ER diagram:
If we map every entity set and relationship to its own table, we get the following table diagram:
However, if we denormalize the relationship tables into the schedule table (since they all have the same primary key), our table diagram changes to:
Compare this with the first table diagram. While these physical models are very similar, they derive from very different conceptual models. Strictly speaking, I think both "entity table" and "relationship table" are inappropriate for the denormalized schedule table. In the network data model, we would call it an associative entity (but that's not the same as associative entities in the ER model).
Finally, relationships can have attributes too:
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
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.
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.
I have two types of accounts (customer and provider), I chose the single-table strategy for persistence. Customer creates Orders (one2many) and provider bids on the orders in auction style (many2many relationship, because he can bid on many orders as well as other providers). My question is, is it possible to have these relationships simultaneously ? Because logically it can work. But MDA code generators don't like it. If so, what drawbacks I could come across with this datamodel.
Thanks in advance.
The disadvantage is that you can't enforce referential integrity in the database between the accountID in the accounts table and the accountID in the bids table (which I assume represents the accountID of the provider bidding on the order) because not all accountID values are allowable.
But, don't give up on the single-table solution for accounts, which may well be the correct one for your problem (I can't say for sure not completely understand the relation between providers and customers). Here's what you need to do to both use the single table solution and allow referential integrity:
Remove isProvider and isCustomer from Accounts.
Add two new tables Providers and Customers. Each table will have an accountID column which is both the primary key in that table and a foreign key back to the original account table.
Migrate any additional columns from Accounts that are unique to either Providers or Customers into the appropriate table.
Now, the accountID in the Orders table should be a foreign key into Customers, not Accounts. Similarly, the accountID column in Bids becomes a foreign key into Providers rather than Accounts.
Relational integrity and single-table storage for accounts is provided for.
"I chose the single-table strategy for persistence" - that's actually not that good a reason for combining them, in my opinion. Customers and providers are fundamentally different beasts.
The fact that you're having troubles is a clear indication that you're most likely doing it the wrong way - that's true of most things in the IT industry (and probably life itself but you don't need me proselytising on that).
I would separate them out into different tables to resolve this particular problem.
If you really want part of the data to be shared, you could put the common things in yet another table and reference it from the customers and providers tables.
You may want this if a single entity can be both a customer and provider - in that case, you would want the two different table entries to share the same information (such as balance, reputation and so on).