title: ARRAY_AGG(STRUCT(x,y,z)) equivalent in Bigquery legacy SQL
body: <p>I have a standard SQL query of the following structure</p>
  
  <pre><code>SELECT a, ARRAY_AGG(STRUCT(x,y,z))
  FROM t
  GROUP BY a
  </code></pre>
  
  <p>How can write the same query in legacy SQL?</p>