In a database, what is the difference betwen a key and an index?

Question

For example, in SQLServer, if I have an index with \'unique\' set on it, how is that different from a key?

How do I know when I want to use a Key vs. an Indexed field?

Answer 1

A key is a field or set of fields that uniquely identifies each row. Typically, there will be one key that is designated as the primary key, and nowadays this seems to be one field that is frequently meaningless by itself (much like a page number, which is important only in identifying what's on the page).

It's possible to have a table with a key for another table, which is called a foreign key. For example, in an online store's database, there may be a table for orders, and each order will have an associated customer, so each row in the orders table has a foreign key for the customer table.

An index is a data structure based on one or more fields in a table, which allows rapid access to the table based on those fields. The index may identify individual rows (for example, an index of the customer email address), or may indicate groups (for example, tax-exempt status of customers). It is used much like the index of a book, or like a reverse directory (such as a phone book ordered by telephone number).

A key is mostly a way to think about the database (although it's usually possible to tell the database about keys, so the database can reject rows with duplicate keys, for example). An index is a way to make the database work faster.