[Beltalowda]

It's a 0-1:1 relationship, so you'd have columns like "name", "email", "car", etc. on the employees table, and "car" might have a unique index (which is probably an ID referencing a "cars" table).

Since employees can only have one car and since a car can only be used by one employee, an unique index is fine, but there may be 20 people that don't have a car so you don't want NULL values to be unique.

Of course there are always ways to work around this, but this is the general idea.