title: error : subquery must return only one column
body: <p>I am getting the error <code>subquery must return only one column</code> when I try to run the following query:</p>
  
  <pre><code>SELECT mat.mat as mat1, sum(stx.total ) as sumtotal1,
    (
      SELECT mat.mat  as mat, sum(stx.total)  as sumtotal
      FROM stx 
        LEFT JOIN mat ON stx.matid = mat.matid
        LEFT JOIN sale ON stx.saleid = sale.id
      WHERE stx.date BETWEEN '2013-05-01' AND '2013-08-31' 
        AND sale.userid LIKE 'A%'
      GROUP BY mat.mat
  ) AS MyField
  FROM stx 
    LEFT JOIN mat ON stx.matid = mat.matid
    LEFT JOIN sale ON stx.saleid = sale.id
  WHERE stx.date BETWEEN '2013-05-01' AND '2013-08-31'
    AND sale.userid LIKE 'B%'
  GROUP BY mat.mat
  </code></pre>
  
  <p>What is causing this error?</p>