Weak relationship database

weak relationship database

We have two types of entities Strong entity and Weak entity. The weak entity is always dependent on the strong entity. Let us discuss the. In your example, RESERVATION is a weak entity which relies on both the FK to CUSTOMER and the FK to VIDEO in addition to a further. The Spouse table, in the COMPANY database, is a weak entity because its primary key is dependent on the Employee table. Without a corresponding employee.

Let us understand this concept with the help of an example; a customer borrows a loan.

weak relationship database

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.

weak relationship database

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.

Weak Entity Modeling

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.

  • Weak entity

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. For the presence of car loan in loan entity set, it must have been borrowed by a customer. In this way, the weak Loan entity is dependent on the strong Customer entity. The second thing, we know is a weak entity does not have a primary key. In the Loan entity set, we have two exactly same entities i.

Weak entity - Wikipedia

So, it will be determined that one home loan is borrowed by Customer Jhon and other by Customer Ruby. This is how the composed primary key of weak entity identify each entity in weak entity set. Key Differences Between Strong Entity and Weak Entity The basic difference between strong entity and a weak entity is that the strong entity has a primary key whereas, a weak entity has the partial key which acts as a discriminator between the entities of a weak entity set.

October Learn how and when to remove this template message In a relational databasea 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.

Difference Between Strong and Weak Entity

The foreign key is typically a primary key of an entity it is related to. In entity relationship diagrams ER diagramsa weak entity set is indicated by a bold or double-lined rectangle the entity connected by a bold or double-lined type arrow to a bold or double-lined diamond the relationship. This type of relationship is called an identifying relationship and in IDEF1X notation it is represented by an oval entity rather than a square entity for base tables.

An identifying relationship is one where the primary key is populated to the child weak entity as a primary key in that entity. In general though not necessarily a weak entity does not have any items in its primary key other than its inherited primary key and a sequence number. There are two types of weak entities: The latter represents a crucial type of normalizationwhere the super-type entity inherits its attributes to subtype entities based on the value of the discriminator. Complete subtype relationship, when all categories are known.

How to identify a strong vs weak relationship on ERD? - Stack Overflow

Incomplete subtype relationship, when all categories may not be known. The standard example of a complete subtype relationship is the party entity. When sub-type relationships are rendered in a database, the super-type becomes what is referred to as a base table. The sub-types are considered derived tables, which correspond to weak entities.

weak relationship database

Referential integrity is enforced via cascading updates and deletes.