title: When to use 1-to-1 relationships between database tables?
body: <p>A DB design question: when do you decide to use 1 to 1 relation tables?</p>
  
  <p>One of the places I see this is, for example, when you have a User and UserProfile table, people split them instead of putting all columns just in a User table.</p>
  
  <p>Technically, you can just put all the columns in one table since their relationship is 1-to-1.  </p>
  
  <p>I know someone said that for the UserProfile table, over time you need to alter table to add more columns, but I really don't think this is a strong reason to split the tables.</p>
  
  <p>So, if I'm to design a User table and UserProfile table, is it better for me to just do it in one table?</p>