Cardinality in DBMS – max/minimum cardinality & examples
Logical Relationship in Tables: Database tables have a major advantage to bridge them with some logical relationship so to filter the records. These relationship allowed us to access one table data using another table records(via columns or rows). In other words logical filtration is called cardinality DBMS.
Read Also: Relational Model in DBMS
Definitions of Cardinality in DBMS:
Definition 1: A number that represents, one instance of first entity(table) is related to how many instances of the second entity, is known as cardinality in dbms.
Definition 2: Cardinality is a relationship or join/bridge between rows of 1 table to the rows of another table. via some column (e.g. User Table column {user_id} in another table Posts to filter the posts by user_id).
Types of Cardinality in DBMS:
There are three types of Cardinality in DBMS.
- one-to-one
- one-to-many
- many-to-one
- many-to-many
Read Also: Degree of Relationship in DBMS
one-to-one Cardinality:
one-to-many Cardinality:
many-to-one Cardinality:
many-to-many Cardinality:
Cardinality Examples in DBMS:
Let’s take an example in order to illustrate the concepts. There are two entities “Person and Address”.
Minimum Cardinality:
Does every Person need to have an Address? If yes then its minimum cardinality is 1 and if no then its minimum cardinality is 0.
Maximum Cardinality:
Can any given Person have more than one Address? If not then maximum cardinality is 1 and in case of yes, its unbounded, means many and is denoted by *.
FAQ’s – Cardinality in DBMS:
The number of entities to which another entity can be associated through relationship. Like a table has product and other table has orders of that particular product. both can be grab by id of that product is called relationship or it can have one to one or many to many or one to many cardinality.
Cardinality represent the number of occurrences in the related entity. If you use boxes for entities in E-R model, then the outside symbols closed to boxes are called the outside cardinalities or maximum. Their symbols can be 1, . And the inside called the inside cardinality or minimum cardinality. Their symbols can be 0 or 1. Yes optional or compulsory is only for minimum cardinality, 0 is used for optional and 1 is for compulsory.
Suppose you have 2 tables where one is salary table and another is users table. If you want to filter the data e.g take out the salary with user details you need to create a bridge between the users id so you can get salary and user details as well.
Use of cardinality is to better understand the business purpose of any system and it explains the data model of any system which help upcoming developers to understand what are the cardinalities and how to customize.
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