title: Database relationship 1:1 vs 1:0..1
body: <p>I'm trying to learn these relationships. Is it correct as I show below that:</p>
  
  <p><strong>1)</strong> I will have a <strong>1:0..1</strong> relation by simply making the identity key of the parent also the identity key of the child?  </p>
  
  <p><strong>2)</strong> To make it <strong>1:1</strong>, I add an FK in parent table, connecting the two Identity columns?  </p>
  
  <p>Is this all there is to it? And please correct me if my wording is not exact/correct.</p>
  
  <pre><code>1:0..1
  </code></pre>
  
  <p><strong>Customer</strong>(Id {PK, Identity}, Name) </p>
  
  <p><strong>CustomerAddress</strong>(CustomerId {PK, FK, Identity}, StreetName)</p>
  
  <p><strong>FK_CustomerAddress_Customer</strong>(CustomerId, Id)</p>
  
  <pre><code>1:1
  </code></pre>
  
  <p><strong>Customer</strong>(Id {PK, FK, Identity}, Name)</p>
  
  <p><strong>FK_Customer_CustomerAddress</strong>(Id, CustomerId)</p>
  
  <p><strong>CustomerAddress</strong>(CustomerId {PK, FK, Identity}, Streetname)</p>
  
  <p><strong>FK_CustomerAddress_Costumer</strong>(CustomerId, Id)</p>
  
  <hr>
  
  <p><strong>EDIT1:</strong></p>
  
  <p>I believe both above was wrong, and solution is like this: (?)  </p>
  
  <pre><code>1:0..1
  </code></pre>
  
  <p><strong>Customer</strong>(Id <em>{PK, Identity}</em>, Name)</p>
  
  <p><strong>CustomerAddress</strong>(CustomerId <em>{PK, FK, UNIQUE}</em>, StreetName)</p>
  
  <p><strong>FK_CustomerAddress_Customer</strong>(CustomerId, Id)</p>