Relationships & Relationship Sets
In relational database theory, a relation, as originally defined by E. F. Codd, is a set of tuples In SQL, a database language for relational databases, relations are represented by tables, where each row of a table represents a single tuple, and. An entity–relationship model (ER model for short) describes interrelated things of interest in a . In other words, a relationship set corresponds to a relation in mathematics, while a relationship corresponds to a member of the relation. DBMS - Relationship and Relationship Sets - DBMS Relationship and Entity And Entity Set, Attribute And Domain, Keys Super, Candidate, Primary, Alternate, .
A view can be defined by an expression using the operators of the relational algebra or the relational calculus. Such an expression operates on one or more relations and when evaluated yields another relation. The result is sometimes referred to as a "derived" relation when the operands are relations assigned to database variables.
A view is defined by giving a name to such an expression, such that the name can subsequently be used as a variable name.
Note that the expression must then mention at least one base relation variable. The following is an example. R is a relation on these n domains if it is a set of elements of the form d1, d2, One reason for abandoning positional concepts altogether in the relations of the relational model is that it is not at all unusual to find database relations, each of which has as many as 50,or even columns. Communications of the ACM.
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No credit card required The Entity Relationship Model At a basic level, databases store information about distinct objects, or entities, and the associations, or relationships, between these entities.
For example, a university database might store information about students, courses, and enrollment. A student and a course are entities, while an enrollment is a relationship between a student and a course. Similarly, an inventory and sales database might store information about products, customers, and sales. A product and a customer are entities, while a sale is a relationship between a customer and a product. A popular approach to conceptual design uses the Entity Relationship ER model, which helps transform the requirements into a formal description of the entities and relationships that appear in the database.
In the ER diagram, an entity set is represented by a rectangle containing the entity name. An entity set is represented by a named rectangle We typically use the database to store certain characteristics, or attributes, of the entities.
In a sales database, we could store the name, email address, postal address, and telephone number for each customer. Attributes describe the entity they belong to. An attribute may be formed from smaller parts; for example, a postal address is composed of a street number, city, ZIP code, and country.
Some attributes can have multiple values for a given entity. For example, a customer could provide several telephone numbers, so the telephone number attribute is multivalued. Attributes help distinguish one entity from other entities of the same type. We could use the name attribute to distinguish between customers, but this could be an inadequate solution because several customers could have identical names.
To be able to tell them apart, we need an attribute or a minimal combination of attributes guaranteed to be unique to each individual customer.
The identifying attribute or attributes form a key. In our example, we can assume that no two customers have the same email address, so the email address can be the key.
ER Model - Basic Concepts
However, we need to think carefully about the implications of our choices. For example, if we decide to identify customers by their email address, it would be hard to allow a customer to have multiple email addresses. Any applications we build to use this database might treat each email address as a separate person, and it might be hard to adapt everything to allow people to have multiple email addresses.
Clearly, there may be several possible keys that could be used to identify an entity; we choose one of the alternative, or candidate, keys to be our main, or primary, key. You usually make this choice based on how confident you are that the attribute will be non-empty and unique for each individual entity, and on how small the key is shorter keys are faster to maintain and use.
Attributes comprising the primary key are shown underlined. The parts of any composite attributes are drawn connected to the oval of the composite attribute, and multivalued attributes are shown as double-lined ovals. Similarly, a product price could be a positive rational number.
Attributes can be empty; for example, some customers may not provide their telephone numbers. You should think carefully when classifying an attribute as multivalued: The sales database requirements may specify that a product has a name and a price.
To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key. Each product entity would have name, price, and product ID attributes.
The ER diagram representation of the product entity Representing Relationships Entities can participate in relationships with other entities. For example, a customer can buy a product, a student can take a course, an artist can record an album, and so on. Like entities, relationships can have attributes: Our database could then record each sale and tell us, for example, that at 3: For example, each customer can buy any number of products, and each product can be bought by any number of customers.
This is known as a many-to-many relationship. We can also have one-to-many relationships. For example, one person can have several credit cards, but each credit card belongs to just one person.
Looking at it the other way, a one-to-many relationship becomes a many-to-one relationship; for example, many credit cards belong to a single person. Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine.
Entity–relationship model - Wikipedia
We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively. The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship.
There are many relationships that may at first seem to be one-to-one, but turn out to be more complex.