Binary relation - Wikipedia
An entity set may contain entities with attribute sharing similar values. For example A set of relationships of similar type is called a relationship set. Like entities. Relationship Set: Collection of similar relationships. – An n-ary relationship set R relates n entity sets E1 En ; each relationship in R involves entities e1 ∈ E1. For instance, PERSON denotes the entities of an entity-set with attributes NAME and AGE, while ASSIGNED denotes the relationships between the entities of.
Matsumoto Yukihiro Okinawa, Okinawa A predicate for this relation, using the attribute names to denote free variables, might be "Employee number ID is known as Name and lives at Address". Examination of the relation tells us that there are just four tuples for which the predicate holds true.
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So, for example, employee is known only by that name, Yonezawa Akinori, and does not live anywhere else but in Naha, Okinawa. Also, apart from the four employees shown, there is no other employee who has both a name and an address.
Under the definition of body, the tuples of a body do not appear in any particular order - one cannot say "The tuple of 'Murata Makoto' is above the tuple of 'Matsumoto Yukihiro'", nor can one say "The tuple of 'Yonezawa Akinori' is the first tuple. Under the definition of heading, the attributes of an element do not appear in any particular order either, nor, therefore do the elements of a tuple.
A similar comment does not apply here to SQL, which does define an ordering to the columns of a table.
Relation Variables[ edit ] A relational database consists of named relation variables relvars for the purposes of updating the database in response to changes in the real world. An update to a single relvar causes the body of the relation assigned to that variable to be replaced by a different set of tuples. 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.
ER Model - Basic Concepts
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 Entity Relationship Model - Learning MySQL [Book]
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.
"Relations" and "Relationship" | Ask The Editor | Learner's Dictionary
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.
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
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.
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. For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity.
Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is. In an ER diagram, we represent a relationship set with a named diamond. The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book.
The ER diagram representation of the customer and product entities, and the sale relationship between them. An entity-set must have a primary entity-identifier and can have several alternative entity-identifiers.Entities, Attributes, and Relationships
Entity-identifiers are not always enough to uniquely distinguish among the instances of an entity-set. For example, a reference may appear in several publications and therefore the entity-identifier NAME of entity-set REFERENCE is not enough to uniquely distinguish between the various instances of references with the same name that have appeared in different publications.
Such entity-sets are called weak, and said to depend for identification ID-dependent on other entity-sets. Then the instances of a weak entity-set, E are uniquely identified using the primary identifier of E together with the primary identifiers of the entity-sets on which E is ID-dependent.
An entity-set that is involved in a relationship-set is said to have a role in that relationship-set. Roles are essential in distinguishing the multiple involvements of an entity-set in a relationship-set. In the EER model attributes are by default single-valued and primitive, that is, they can be associated only with primitive value-sets and cannot take values from entity-sets, that is, cannot be defined as abstract.