title: Invalid use of null value
body: <p>I have a problem using mySQL.</p>
  
  <p>This error pops up:</p>
  
  <p><code>Invalid use of null value</code></p>
  
  <p>I was trying to make two attributes in a table the primary key; here is my code: </p>
  
  <pre><code>alter table contact_info
  add primary key(phone_number, contactID);
  </code></pre>
  
  <p>Here are the alter statements I put into my contact_info table:</p>
  
  <pre><code>alter table contact_info
  add contactID varchar(10);
  
  alter table contact_info
  add suffixID varchar(8);
  
  alter table contact_info
  add titleID varchar(12);
  
  alter table contact_info
  add companyID varchar(12);
  
  alter table contact_info
  add phonetypeID char(2);
  </code></pre>
  
  <p>Does anybody know what's wrong? Thanks in advance.</p>