Uniqueness : (唯一獨特）
It is Uniqueness attribute in table of users .
Your passport , Student ID , Health Card ID , Drive Card ID
Before have say attribute of Drive Card ID , Right ?
Some Students NO drive card , but have students card and Health card .
The driver card ID at this point is not valid what is Pramary Key .
e.g : which is not unqueness ? name , name ID , e-mail , cell phone ,??? A: name . explain : a lot people same name . so it's not comfort Availability
It’s used to build( establish) relation between data .
In Table of Foreign , absolutions from others table of Primary .
the car_table be foreign key = call car_for.
name_table be foreign key = name_ for
which is foreign key ? answer : car_table of name for is foreign key . also foreign key is Primary key .