Difference between Strong Entity and Weak Entity
An entity is represented by a set of attributes, that is, . Diamonds represent relationship sets. key of the strong entity set on which the weak entity set is. Weak Entity Sets. Converting E/R ◇In an entity-relationship diagram: ◇ Entity set a line to the rectangle representing its entity set. .. Example: Good. Beers. In the ER diagram, both the weak entity and its corresponding relationship are represented using a double line and the partial key is underlined.
It is denoted by a single rectangle. A strong entity always has the primary key in the set of attributes that describes the strong entity. It indicates that each entity in a strong entity set can be uniquely identified. Set of similar types of strong entities together forms the Strong Entity Set.
A strong entity holds the relationship with the weak entity via an Identifying Relationship, which is denoted by double diamond in the ER diagram. On the other hands, the relationship between two strong entities is denoted by a single diamond and it is simply called as a relationship.
Let us understand this concept with the help of an example; a customer borrows a loan. Here we have two entities first a customer entity, and second a loan entity.
But even if a customer does not borrow any loan it would be listed in Customer entity set.
The Entity Relationship Model - Learning MySQL [Book]
So we can conclude that a customer entity does not depend on a loan entity. This makes Customer entity a strong entity on which a loan entity depends. Definition of Weak Entity A Weak entity is the one that depends on its owner entity i. A weak entity is denoted by the double rectangle. Weak entity do not have the primary key instead it has a partial key that uniquely discriminates the weak entities. The primary key of a weak entity is a composite key formed from the primary key of the strong entity and partial key of the weak entity.
The collection of similar weak entities is called Weak Entity Set. The relationship between a weak entity and a strong entity is always denoted with an Identifying Relationship i. We have Loan as our weak entity, and as I said above for each loan there must be at least one borrower. You can observe in the loan entity set, no customer has borrowed a car loan and hence, it has totally vanished from loan entity set.Database [ DBMS ] - STRONG entity Vs WEAK entity
For the presence of car loan in loan entity set, it must have been borrowed by a customer. 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. 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. Partial and Total Participation Relationships between entities can be optional or compulsory.
In our example, we could decide that a person is considered to be a customer only if they have bought a product. On the other hand, we could say that a customer is a person whom we know about and whom we hope might buy something—that is, we can have people listed as customers in our database who never buy a product.
These are referred to as the participation constraints of the relationship. In an ER diagram, we indicate total participation with a double line between the entity box and the relationship diamond.
From time to time, we encounter cases where we wonder whether an item should be an attribute or an entity on its own. For example, an email address could be modeled as an entity in its own right.
When in doubt, consider these rules of thumb: Is the item of direct interest to the database? Objects of direct interest should be entities, and information that describes them should be stored in attributes. Our inventory and sales database is really interested in customers, and not their email addresses, so the email address would be best modeled as an attribute of the customer entity. Does the item have components of its own? If so, we must find a way of representing these components; a separate entity might be the best solution.
In the student grades example at the start of the chapter, we stored the course name, year, and semester for each course that a student takes. Can the object have multiple instances? If so, we must find a way to store data on each instance.
The cleanest way to do this is to represent the object as a separate entity. In our sales example, we must ask whether customers are allowed to have more than one email address; if they are, we should model the email address as a separate entity.
Is the object often nonexistent or unknown? If so, it is effectively an attribute of only some of the entities, and it would be better to model it as a separate entity rather than as an attribute that is often empty. Consider a simple example: The ER diagram representation of student grades as a separate entity Entity or Relationship?
An easy way to decide whether an object should be an entity or a relationship is to map nouns in the requirements to entities, and to map the verbs to relations. All else being equal, try to keep the design simple, and avoid introducing trivial entities where possible; i. Intermediate Entities It is often possible to conceptually simplify many-to-many relationships by replacing the many-to-many relationship with a new intermediate entity sometimes called an associate entity and connecting the original entities through a many-to-one and a one-to-many relationship.
A passenger participates in an M: Any given flight can have many passengers with a booking. Any given passenger can have bookings on many flights. Hence, we can consider the many-to-many relationship to be in fact two one-to-many relationships, one each way. This points us to the existence of a hidden intermediate entity, the booking, between the flight and the passenger entities. The requirement could be better worded as: The intermediate booking entity between the passenger and flight entities Each passenger can be involved in multiple bookings, but each booking belongs to a single passenger, so the cardinality of this relationship is 1: Similarly, there can be many bookings for a given flight, but each booking is for a single flight, so this relationship also has cardinality 1: Since each booking must be associated with a particular passenger and flight, the booking entity participates totally in the relationships with these entities.
We described partial and total participation earlier in Partial and Total Participation.