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Mapping constraints in DBMS

By Chaitanya Singh | Filed Under: DBMS

Mapping constraints can be explained in terms of mapping cardinality:

Mapping Cardinality:
One to One: An entity of entity-set A can be associated with at most one entity of entity-set B and an entity in entity-set B can be associated with at most one entity of entity-set A.

One to Many: An entity of entity-set A can be associated with any number of entities of entity-set B and an entity in entity-set B can be associated with at most one entity of entity-set A.

Many to One: An entity of entity-set A can be associated with at most one entity of entity-set B and an entity in entity-set B can be associated with any number of entities of entity-set A.

Many to Many: An entity of entity-set A can be associated with any number of entities of entity-set B and an entity in entity-set B can be associated with any number of entities of entity-set A.

We can have these constraints in place while creating tables in database.

Example:

CREATE TABLE Customer (
customer_id int PRIMARY KEY NOT NULL,
first_name varchar(20),
last_name varchar(20)
);

CREATE TABLE Order (
order_id int PRIMARY KEY NOT NULL,
customer_id int,
order_details varchar(50),
constraint fk_Customers foreign key (customer_id) 
       references dbo.Customer
);

Assuming, that a customer orders more than once, the above relation represents one to many relation. Similarly we can achieve other mapping constraints based on the requirements.

Comments

  1. mak says

    January 6, 2016 at 6:12 AM

    You provided a very good material on DBMS. I really appreciate that.

    Thank you,

    Reply

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