Posts Tagged ‘Basic Constructs’


Entity Relationship Diagram (ERD) — Part-II

In Database Systems on June 16, 2012 by Muhammad Haadi Tagged: , , , , , , , , ,

Cardinality of a Relationship

Cardinality is the mapping of entities i-e zero, one or many. It basically explains how a table is linked to another table. It can be particularized more as the number of distinct values of a table connected to how many values of the other table – both minimum and maximum.

NOTE: Some authors explain optionality as a separate concept which is zero or one in this case.

Following are the two possible classifications of relations among entities.

  • One to One
  • One to Many

There is also a third case of (Many to Many) relationship but I consider it separate from the two basic classifications as it is basically resolved with the help of the same technique used in resolving (one to many) relationship. We will see that in detail later.

We will first discuss the One-to-Many and Many-to-Many  scenario as the example I am going to use is the One-to-One relation would need an understanding of one-to-many relation.

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Post Entity Relationship Diagram (ERD) – Part-I

In Database Systems on January 13, 2012 by Muhammad Haadi Tagged: , , , , , , ,

This topic has always been a challenge for me to discuss. It’s not that elaborating the concept among a bunch of students bothers me but the thing is that normally students get confused in the twists and turns this topic has.

However, I am going to keep things simple by using the divide and conquer approach, i-e. I will segregate the two main portions involved in this topic

  • the basics of ERD and
  • the one with relations explained

Therefore, that would make the concepts quite easy to understand and learn.

Note:There are various explanations of these concepts over the internet as some authors keep different concepts isolated to make them more understandable while others try not to indulge a newbie into the details. So, if you feel difficulty anywhere, feel free to ask. Read More »

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