title: MySql Subquery returns more than 1 row
body: <p>im having some problems to query at mysql a self many to many relationship
  i got a Person table and a Friendship table</p>
  
  <p>Person<br>
  pid | name | age<br>
  1 Jimmy 18<br>
  2 Lucas 25<br>
  3 Marie 12<br>
  4 Marie 40<br>
  5 John 30  </p>
  
  <p>Friendship<br>
  pid | pId2<br>
  1 3<br>
  2 3<br>
  4 1<br>
  5 3</p>
  
  <hr>
  
  <p>So i need to check whats Marie's friends (correct answer: Lucas and John )</p>
  
  <p>with the follow query i got the answer by the pid</p>
  
  <pre><code>SELECT p.name
  FROM Person p
  WHERE EXISTS(SELECT NULL
      FROM Friendship f
      WHERE p.pid = f.pid
      AND (f.pid2 = 4 OR f.pid2 = 3))
  OR EXISTS(SELECT NULL
      FROM Friendship f
      WHERE p.pid = f.pid2
      AND (f.pid = 4 OR f.pid = 3))  
  </code></pre>
  
  <p>but what if i need to query by the name?the follow one doesnt work</p>
  
  <pre><code>SELECT p.name
  FROM Person p
  WHERE EXISTS(SELECT NULL
      FROM Friendship f
      WHERE p.pid = f.pid
      AND f.pid2 = (SELECT pid FROM Person WHERE name = 'Marie'))
  OR EXISTS(SELECT NULL
      FROM Friendship f
      WHERE p.pid = f.pid2
      AND f.pid = (SELECT pid FROM Person WHERE name = 'Marie')) 
  </code></pre>
  
  <p>thanks for your help guys</p>