Mapping constraints in 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.