In a relational database, a weak entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to.
An attribute is defined as a quality or characteristic of a person, place, or thing. There is a difference between attributes and traits, but it is slight and some characteristics could be considered either an attribute or a trait.
Examples of an entity are a single person, single product, or single organization. Entity type. A person, organization, object type, or concept about which information is stored. A characteristic or trait of an entity type that describes the entity, for example, the Person entity type has the Date of Birth attribute.
Weak relationship. Weak Relationships are connections between a weak entity and its owner.
For example, ONE team has MANY players. When present in an ERD, the entity Team and Player are inter-connected with a one-to-many relationship. In an ER diagram, cardinality is represented as a crow's foot at the connector's ends. The three common cardinal relationships are one-to-one, one-to-many, and many-to-many.
Definition of Strong EntityThe Strong Entity is the one whose existence does not depend on the existence of any other entity in a schema. 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.
'Can a weak entity be related to another weak entity' I would say No. A weak entity has no primary key of its own but does have a foreign key which is linked to the PK of another entity. Thus if you have two weak entities with the same FK id they are infact related to the same strong entity rather than each other.
An entity can be of two types: Tangible Entity: Tangible Entities are those entities which exist in the real world physically. Example: Person, car, etc. Intangible Entity: Intangible Entities are those entities which exist only logically and have no physical existence.
As the weak entities do not have any primary key, they cannot be identified on their own, so they depend on some other entity (known as owner entity). The weak entities have total participation constraint (existence dependency) in its identifying relationship with owner identity. Weak entity types have partial keys.
For each weak entity in the model-there are three: inventory, order, and item-translate the entity directly to a CREATE TABLE statement as in Step 1. Include all attributes as in Step 1. Include as attributes the primary key attributes of the owning entity; that is, the entity the weak entity is related to.
A partial key means just part of a key - some proper subset of the key attributes. In your example if the primary key of a Child was (Empid, ChildName) with Empid as a foreign key referencing the Employee then Child is a weak entity.
While converting weak entity set into relation schema, the following procedure is followed; Take the Primary key of Strong entity set in which the Weak entity set depends. Then include the primary key into the relation schema of weak entity set along with other attributes of weak entity set.
Composite Attribute –An attribute composed of many other attribute is called as composite attribute. For example, Address attribute of student Entity type consists of Street, City, State, and Country. In ER diagram, composite attribute is represented by an oval comprising of ovals.
A relational database is a collection of data items with pre-defined relationships between them. These items are organized as a set of tables with columns and rows. Tables are used to hold information about the objects to be represented in the database.
ERD
| Acronym | Definition |
|---|
| ERD | Emergency Recovery Disk |
| ERD | Eclipse Resource Database |
| ERD | Electronic Report Distribution |
| ERD | Event-Related Desynchronization |
A relationship type is the type of association between two or more entities. For example teacher “teaches” student, where teacher and student are two entities and the relationship type between them is “teaches”. The association among 2 or more entities is called relationship.
A binary relationship is when two entities participate and is the most common relationship degree. A unary relationship is when both participants in the relationship are the same entity.
Identifying relationships exist when the primary key of the parent entity is included in the primary key of the child entity. On the other hand, a non-identifying relationship exists when the primary key of the parent entity is included in the child entity but not as part of the child entity's primary key.
Keys. Foreign key: term used in relational databases (but not in the E-R model) for an attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.
Cardinality means two things in databases. In this sense, cardinality means whether a relationship is one-to-one, many-to-one, or many-to-many. So you're really talking about the relationship cardinality. Cardinality's official, non-database dictionary definition is mathematical: the number of values in a set.
Derived attribute − Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived.
Weak Entities
| Strong Entity Set | Weak Entity Set |
|---|
| Primary Key is one of its attributes which helps to identify its member. | In a weak entity set, it is a combination of primary key and partial key of the strong entity set. |
