title: Scalar subquery contains more than one row
body: <p>Im working with H2 database and wanted to move some data. For that I created the following Query:</p>
  
  <pre><code>UPDATE CUSTOMER
  SET EMAIL = SELECT service.EMAIL
  FROM CUSTOMER_SERVICE AS service
  INNER JOIN CUSTOMER AS customer ON service.ID = customer.CUSTOMER_SERVICE_ID;
  </code></pre>
  
  <p>When I now perform it in the H2 console I get the following error:</p>
  
  <pre><code>Scalar subquery contains more than one row; SQL statement:
  
  UPDATE CUSTOMER
  SET EMAIL = SELECT service.EMAIL
  FROM CUSTOMER_SERVICE AS service
  INNER JOIN CUSTOMER AS customer ON service.ID = customer.CUSTOMER_SERVICE_ID [90053-192] 90053/90053 (Hilfe)
  </code></pre>
  
  <p>What is this error telling me?</p>
  
  <p><strong>EDIT</strong> </p>
  
  <p>What I want to achiev with my query:</p>
  
  <p>Actually every <code>CUSTOMER</code> has a <code>CUSTOMER_SERVICE</code>. And I simply want to move the <code>COLUMN EMAIL</code> from <code>CUSTOMER_SERVICE</code> to the <code>CUSTOMER</code> Table. for that I already added a email column to the user. I hoped to be able to do it with my query but obviously not.</p>