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    <p>The relational model says there's no essential difference between one key and another. That is, when a relation has more than one candidate key, there are no theoretical reasons for declaring that <em>this</em> key is more important than <em>that</em> key. Essentially, that means there's no theoretical reason for identifying one key as a <em>primary key</em>, and all the others as secondary keys. (There might be practical reasons, though.)</p> <p>Many relations have more than one candidate key. For example, a relation of US states might have data like this.</p> <pre><code>State Abbr Postal Code -- Alabama Ala. AL Alaska Alaska AK Arizona Ariz. AZ ... Wyoming Wyo. WY </code></pre> <p>It's clear that values in each of those three columns are unique--there are three candidate keys. </p> <p>If you were going to build a table in SQL to store those values, you might do it like this.</p> <pre><code>CREATE TABLE states ( state varchar(15) primary key, abbr varchar(10) not null unique, postal_code char(2) not null unique ); </code></pre> <p>And you'd do something like that because SQL doesn't have any other way to say "My table has three separate candidate keys."</p> <p>I didn't have any particular reason for choosing "state" as the primary key. I could have just as easily chosen "abbr" or "postal_code". Any of those three columns can be used as the target for a foreign key reference, too.</p> <p>And as far as that goes, I could have built the table like this, too.</p> <pre><code>CREATE TABLE states ( state varchar(15) not null unique, abbr varchar(10) not null unique, postal_code char(2) not null unique ); </code></pre>
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    1. USMike Sherrill 'Cat Recall'
    1. USMike Sherrill 'Cat Recall'
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    1. COIn fact, most if not all database products will let you create relations off unique constraints. Thus, in theory, you could have used a unique constraint on all three. Technically, ISO could have used the words "unique key" instead of just "unique" but that would have been redundant.
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      1. USThomas
    2. COThanks. This is helpful. So what does it mean when the book says `UNIQUE KEY`
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      1. USlocoboy
    3. CO+1: Great example.
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      1. USAndrew Lazarus
 

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