title: Subquery returns more than 1 row with update
body: <p>I have been wrestling this issue for hours to no avail. I've been looking at many of the stackoverflow.com questions related to subqueries returning more than one row, but have not been able to find one that helps in my case.</p>
  <p>I have two tables: <strong>clients</strong> and <strong>positions</strong>. I am trying to UPDATE <strong>positions.client_id</strong> with <strong>clients.id</strong> WHERE <strong>clients.file_name</strong> = <strong>positions.file_name</strong>.</p>
  <p>Basically I have a list of positions that need to be assigned to clients. Every client has multiple positions, but every position is assigned to exactly one client. In other words, <strong>client_id</strong> is not unique in <strong>positions</strong>, but <strong>id</strong> is unique (primary key) in <strong>clients</strong>.</p>
  <p>Here's what I have been trying different variations with:</p>
  <pre><code>UPDATE positions 
  SET client_id = (SELECT clients.id 
  FROM clients 
  WHERE clients.file_name = positions.file_name)
  </code></pre>
  <p>It returns.</p>
  <blockquote>
  <h1>1242 - Subquery returns more than 1 row</h1>
  </blockquote>
  <p>Thanks in advance for any help!</p>