How do you represent minimum and maximum cardinalities with ER diagrams?

1 What are cardinalities?

Cardinalities are the numerical values that describe the degree of association between two entities in a relationship. They can be expressed as one-to-one, one-to-many, many-to-one, or many-to-many, depending on how many instances of each entity can participate in the relationship. For example, a one-to-one relationship means that each instance of entity A can be related to only one instance of entity B, and vice versa. A one-to-many relationship means that each instance of entity A can be related to many instances of entity B, but each instance of entity B can be related to only one instance of entity A.

2 How to show cardinalities with ER diagrams?

There are different ways to show cardinalities with ER diagrams, but one of the most common methods is to use the crow's foot notation, which uses symbols at the ends of the relationship lines to indicate the type and number of entities involved. The crow's foot notation uses three symbols: a dash, a circle, and a crow's foot. A dash means one, a circle means zero or optional, and a crow's foot means many or mandatory. By combining these symbols, you can represent the minimum and maximum cardinalities of each entity in a relationship. For example, a dash and a crow's foot at the end of a line means one and only one, while a circle and a crow's foot means zero or more.

3 How to read cardinalities with ER diagrams?

To read cardinalities with ER diagrams, you need to look at the symbols at both ends of the relationship line and interpret them from the perspective of each entity. For example, if you have a relationship between customer and order, and the line has a dash and a crow's foot at the customer end and a circle and a dash at the order end, you can read it as: each customer must place one or more orders, and each order may or may not belong to a customer. Alternatively, you can use the (min, max) notation to express the same cardinalities as: (1,N) for customer and (0,1) for order.

4 How to choose cardinalities with ER diagrams?

To choose cardinalities with ER diagrams, you need to analyze the business rules and requirements of your system and determine how the entities are related to each other. You need to ask questions such as: how many instances of each entity exist in the system? How many instances of each entity are required or optional in a relationship? How many instances of each entity can be associated with another entity in a relationship? Based on the answers, you can select the appropriate symbols and numbers to represent the cardinalities with ER diagrams.

5 How to validate cardinalities with ER diagrams?

To validate cardinalities with ER diagrams, you need to check if the data model accurately reflects the reality and logic of your system. You need to verify if the cardinalities are consistent, complete, and correct. You can use various techniques to validate cardinalities, such as: reviewing the data model with stakeholders and users, testing the data model with sample data and queries, and comparing the data model with existing or similar systems. By validating cardinalities, you can ensure that your data model is reliable and effective.