What is key after all?

[dullboy]

I read some books on relational databases. They talk about primary key and foreign key but my question is what is exactly key? Is there any other key than primary key and foreign key? What is the relationship between key and index? Thanks.

[erickwidya]

..what is exactly key? i define KEY as unique value which means doesn't duplicate. maybe u Passport number that ONLY belongs to u.

Is there any other key than primary key and foreign key? hm..candidate key? (key that can be 'promoted' to Primay Key) but it's only in database concept, composite key? (an unique key that 'made' more than 1 or more key)..

What is the relationship between key and index? index is only for faster searching purpose i think..u may thinking like book that have index at back part..it contain keyword so that ur search being faster rather than use List of Content..

hope u understand..