Let's take a look at the account_history table in the car web portal example. The unique constraint, UNIQUE (account_id, search_key, search_date), has two purposes. The first purpose is to define the validity constraint for inserting the search key into the table only once each day, even if the user searches for the key several times. The second purpose is to retrieve data quickly. Let's assume that we would like to show the last 10 searches for a user. The query for performing this would be as follows:
SELECT search_key FROM account_history WHERE account_id = <account> GROUP BY search_key ORDER BY max(search_date) limit 10;
The preceding query returns only 10 records containing the different search_key ordered by search_date ...
