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Imagine a database for a video rental chain. It probably has hundreds of thousands of customer records. However, when it is time to check out a video, it is important for each customer record to be quickly located for better customer service.
If the video store clerk knows the customer account number, the customer's record is found quickly because the Primary Key index references a unique customer number.
Often the customer account number is not available. But, a customer record is found quickly if the video store database has another unique index that points to the Customer Last Name and Customer Phone columns.
Alternatively, the video store may also want to look up a customer record by last name, even though the search may produce multiple records.
In this case, a non-unique index may be created on the CUSTOMER table that points to the Customer Last Name column alone.
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