Strong and weak entity relationship data

Difference between Strong Entity and Weak Entity

strong and weak entity relationship data

customer entity relationship set account entity Generally, most relationship sets in a database . key of the strong entity set on which the weak entity set is. 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. In a relational database, a weak entity is an entity that cannot be uniquely identified by its In entity relationship diagrams (ER diagrams), a weak entity set is indicated by a bold (or double-lined) rectangle (the entity) connected by a bold (or.

Let us consider another scenario, where we want to store the information of employees and their dependents. The every employee may have zero to n number of dependents. Every dependent has an id number and name. Now let us consider the following data base: There are three employees having E as 1, 2, and 3 respectively.

Database Management System 11 Strong Entity vs Weak Entity

Employee having E 1, has two dependents as 1, Rahat and 2, Chahat. Employee having E 2, has no dependents. Employee having E 3, has three dependents as 1, Raju; 2, Ruhi; 3 Raja.

  • Difference between Strong Entity and Weak Entity
  • Weak entity

Now, in case of Dependent entity id cannot act as primary key because it is not unique. Thus, Dependent is a weak entity set having id as a discriminator. It has a total participation with the relationship "has" because no dependent can exist without the employees the company is concerned with employees.

database - Identifying strong and weak entities in a is-a relationship - Stack Overflow

One of the tables could be called Orders and it would have an order number primary key to identify this order uniquely, and would contain a customer number foreign key to identify who the products are being sold to, plus other information such as the date and time when the order was placed, how it will be paid for, where it is to be shipped to, and so on.

The other table could be called OrderItem; it would be identified by a compound key consisting of both the order number foreign key and an item line number; with other non-primary key attributes such as the product number foreign key that was ordered, the quantity, the price, any discount, any special options, and so on.

There may be zero, one or many OrderItem entries corresponding to an Order entry, but no OrderItem entry can exist unless the corresponding Order entry exists.

strong and weak entity relationship data

The zero OrderItem case normally only applies transiently, when the order is first entered and before the first ordered item has been recorded.

The OrderItem table stores weak entities precisely because an OrderItem has no meaning independent of the Order. Some might argue that an OrderItem does have some meaning on its own; it records that at some time not identified by the record, somebody not identified by the record ordered a certain quantity of a certain product. This information might be of some use on its own, but it is of limited use. For example, as soon as you want to find seasonal or geographical trends in the sales of the item, you need information from the related Order record.

Difference Between Strong and Weak Entity

An order would not exist without a product and a person to create the order, so it could be argued that an order would be described as a weak entity and that products ordered would be a multivalue attribute of the order.

Depends The Strong entity is independent of any other entity in a schema.

strong and weak entity relationship data

Weak entity depends on the strong entity for its existence. Denoted Strong entity is denoted by a single rectangle.

Weak entity is denoted with the double rectangle. Relation The relation between two strong entities is denoted by a single diamond simply called relationship.

What are Strong and Weak Entity Sets in DBMS

The relationship between a weak and a strong entity is denoted by Identifying Relationship denoted with double diamond. Participation Strong entity may or may not have total participation in the relationship. Weak entity always has total participation in the identifying relationship shown by double line.

Definition of Strong Entity The 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.

strong and weak entity relationship data

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.