Database Questions Help?

Question

What's the difference between primary, composite and foreign keys and when are they chosen?

Also, why are queries important to database users?

Answer 1

It's more involved than you can answer in a paragraph. Take a look at this article: http://www.databasedev.co.uk/primary_foreign_keys.html

it explains the differences well. Queries are important to DB users because thats how information is retrieved from the database.

SELECT name, pay FROM employees WHERE pay > 8

is an example of a query that would return a list of employee's names and their hourly pay if they make more than $8/hr. from a database table.

Answer 2

primary keys are the important one in database....which is used for making an ordered list.....which'ill help ti ident..a indivi..one frm bulk queries.....foreign keys r nothing but they are the primary key of an query at the same time it'll be a part of an another query...

Answer 3

Good thing you didn't add "Candidate keys" to the mix, because that's a tough one to answer IMHO. Then we get into whole idea of FD and stuff.

Primary key is one attribute in a tuple that identifies such tuple as unique to other tuples in the same relation.

Composite key is a primary key that happens to be using a combination of two or more attributes instead of one.

Foreign key is less used as implementation but still important in design and writing correct queries; it is a key of either one of above type in attribute, but instead of identifying itself this refers to another tuple of any relation as having a relationship to the tuple that has it; a foreign key in one and a primary key at the other side.