In an e-r diagram, there are/is ________ business rule(s) for every relationship.

An entity type should have a key attribute which uniquely identifies each entity in the entity set, but there exists some entity type for which key attribute can’t be defined. These are called Weak Entity type. 

The entity sets which do not have sufficient attributes to form a primary key are known as weak entity sets and the entity sets which have a primary key are known as strong entity sets. 

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. Partial Keys are set of attributes with the help of which the tuples of the weak entities can be distinguished and identified. 

Note – Weak entity always has total participation but Strong entity may not have total participation. 

Weak entity is depend on strong entity to ensure the existence of weak entity. Like strong entity, weak entity does not have any primary key, It has partial discriminator key. Weak entity is represented by double rectangle. The relation between one strong and one weak entity is represented by double diamond. 

Weak entities are represented with double rectangular box in the ER Diagram and the identifying relationships are represented with double diamond. Partial Key attributes are represented with dotted lines. 

Example-1: 
In the below ER Diagram, ‘Payment’ is the weak entity. ‘Loan Payment’ is the identifying relationship and ‘Payment Number’ is the partial key. Primary Key of the Loan along with the partial key would be used to identify the records. 

Example-2: 
The existence of rooms is entirely dependent on the existence of a hotel. So room can be seen as the weak entity of the hotel. 

Example-3: 
The bank account of a particular bank has no existence if the bank doesn’t exist anymore. 

Example-4: 
A company may store the information of dependents (Parents, Children, Spouse) of an Employee. But the dependents don’t have existence without the employee. So Dependent will be weak entity type and Employee will be Identifying Entity type for Dependent. 

Other examples: 
 

Strong entity | Weak entity
Order | Order Item
Employee | Dependent
Class | Section
Host | Logins 

Note – Strong-Weak entity set always has parent-child relationship.
 

The physical data model is the most granular level of entity-relationship diagrams, and represents the process of adding information to the database. Physical ER models show all table structures, including column name, column data type, column constraints, primary key, foreign key, and relationships between tables.

As shown below, tables are another way of representing entities. The key parts of Entity-relationship Tables are:

Fields

Fields represent the portion of a table that establish the attributes of the entity. Attributes are typically thought of as columns in the database that the ERD models.

In the image above, InterestRate and LoanAmount are both attributes of the entity that are contained as fields.

Keys

Keys are one way to categorize attributes. ER diagrams help users to model their databases by using various tables that ensure that the database is organized, efficient, and fast. Keys are used to link various tables in a database to each other in the most efficient way possible.

Primary Keys

Primary keys are an attribute or combination of attributes that uniquely identifies one and only one instance of an entity.

Foreign Keys

Foreign keys are created any time an attribute relates to another entity in a one-to-one or one-to-many relationship.

Each car can only be financed by one bank, therefore the primary key BankId from the Bank table is used as the foreign key FinancedBy in the Car table.  This BankID is able to be used as the foreign key for multiple cars.

Types

Types refer to the type of data in the corresponding field in a table. Types can also refer to entity types, which describe the composition of an entity; e.g., a book's entity types are author, title, and published date.

How many business rules are there in ER diagram?

What is a business rule quizlet?

What is an ER diagram quizlet?

What four conditions should exist in order to convert a relationship to an associative entity?