ER Diagram interpretation - Certification

[Anonymous]

Hi All,

I am trying to get understanding of ER diagrams and I kind of understand most of the concepts like crow foot notations and cardinality but I have one doubt and I am not finding answer to it.

How does the cardinality affects the number of rows in two tables . for example is to okay to say that If table A and table B have 1:M cardinality then table B must always have more number of rows than A. I am trying to understand the ER models in depth. Please refer any books/material which has scenario based ER diagrams MCQs .

Thanks,

Neha

[swuehl]

I would say table B can have more number of rows than A.

An instance of your model may show only as much rows in table B as in table A.

For example, you can model Customer to Orders using a 1:n relation.

But for a specific instance, there might be only customers with exactely one order.

[Anonymous]

Thanks for the response Swuehl. It makes sense.

Would you be able to suggest any books for ER diagrams study and interpretation practice case studies?

[Gysbert_Wassenaar · Accepted solution]

There are a number of different notations for ERD diagrams. 1:M usually means each record in the left table (the 1) has 0 or more records in the right table (the M). But depending on the ERD notation used it could also mean 1 or more instead of 0 or more. In both cases it does not mean that table B must always have more records than table A. with M being 0 or larger table B could have less records than table A. With M being 1 or larger table B could have the same number of records as table A.

Perhaps this article helps: Entity Relationship Diagram - Common ERD Symbols and Notations

[Anonymous]

Thanks for the clarification, Gysbert. The link you shared is quite informative.