Difference between one to one and one and only one relationship in SQL Server

Hi siddangoud,

If one department can have one employee then, department to employee is a one to one relationship, while if one department can have multiple employees then, department to employee is a one to many relationship.

Thats means table has one row that is associated with one row in another table.

hi

ref this

https://softwareengineering.stackexchange.com/questions/345709/erd-many-vs-zero-or-many-one-or-many-crowfoot-notation

explain me third one 

i e one and only one

only one means lets say

Student can have one and only one login (e.g. user123) and that login can only be assigned to Student1.

When Student1 graduates, no one else can be assigned the login user123.

one means lets say

A student Student1 can only have one room at a time. A room can only have one student at a time.

Next year, Student1 will be assigned a new room and at that point his/her room from this year will be assigned to a new student.