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    <p>It is because this way, the same e-mail address can be used by multiple Persons as well as the same Person can use multiple e-mail addresses, i.e., it makes the relationship between person and e-mail address many-to-many.</p> <p>If the need was merely to enforce that the e-mail address should belong to a person, it would have been enough to make it a foreign key and the column <code>BusinessEntityID</code> not nullable.</p> <p><strong>Update:</strong></p> <p>There are 2 tables involved here, <code>Person</code> and <code>EmailAddress</code>.</p> <p>Each record in <code>Person</code> is identified by a <code>BusinessEntityID</code>. To associate a record from <code>Person</code> with a record in another table <code>T</code>, it is enough to include a column in this table <code>T</code> that refers to <code>BusinessEntityID</code>. Now, if we needed to ensure that all records in <code>T</code> <em>must</em> be associated with some record in <code>Person</code>, then we would place a foreign key constraint on <code>T.BusinessEntityID</code> and make it not nullable. If on top of this, we wanted to ensure that each record in <code>T</code> must be associated with one and only one record in <code>Person</code>, then we could place a uniqueness constraint on the column <code>T.BusinessEntityID</code>.</p> <p>When we make 2 columns, <code>A</code> and <code>B</code> part of the primary key of a table, we are telling the database that the values of these two columns <strong>together</strong> must be unique for all records in that table. It has no bearing on the values in each of those columns and any foreign key relationships.</p> <p>To illustrate:</p> <pre><code>Person (BusinessEntityID, Name) and PK is BusinessEntityID --------------- 1 | John --------------- 2 | Jane --------------- 3 | Sales Team EmailAddress (BusinessEntityID, EmailAddressID, EmailAddress) and PK is [Business EntityID, EmailAddressID] where EmailAddress is auto-incremented -------------- 1 | 1 | john@example.com ------------------------ 1 | 2 | john@contoso.com ------------------------ 2 | 3 | jane@example.com ------------------------ 2 | 4 | jane@contoso.com ------------------------ 1 | 5 | sales@example.com ------------------------ 2 | 6 | sales@example.com ------------------------ 3 | 7 | sales@example.com </code></pre> <p>Data similar to the above is possible to put in the tables in your example. Now what is happening here?</p> <p>There are 3 entities, John, Jane and Sales Team.</p> <p>John has 2 personal e-mail addresses. Jane also has 2 personal e-mail addresses. In addition, the e-mail <code>address sales@example.com</code> belongs to the Sales Team but also to John and Jane.</p> <p>This is a many-to-many relationship.</p> <p>Additonally, if the composite key in EmailAddress is clustered, the keys are stored in the order in which they appear. Read <a href="https://stackoverflow.com/questions/389348/should-a-composite-primary-key-be-clustered-in-sql-server">this</a> for more information.</p>
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    1. POconcept of composite primary key in Adventure Work Database
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    1. USCommunity
    1. USUmar Farooq Khawaja
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    1. COthanks for you reply but if we didn't use BusinessEntityID as a primary key(just we have had EmailAddressID) I think still we could have same address for multiple person and vice versa. am i right?
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      1. USMohammad Yousefi
    2. COis it correct that we think, related two fields in a table as a many to many relationship? because if BusinessEntityID in EmailAddress wasn't primary key(was just a FK), we still would have many mail for one person and many person for a same mail.
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      1. USMohammad Yousefi
    3. COIn fact you are absolutely correct about that. I actually cannot think of a good reason why they have constructed the `EmailAddress` table's PK this way. Usually If you have two tables `A` with `A.Id` and `B` with `B.Id`, and you want a many-to-many relationship between `A` and `B`, you make a third table `C` with `C.AId` and `C.BId` and you can make `(AId, BId) the PK for `C` and also set up FKs for `AId` and `BId` to enforce referential integrity. That's clearly not what's going on here. My aplogies for misguiding you in my answer.
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