title: impala - how to convert a delimited column to multiple rows
body: <p>The value of column scores are delimited by comma. Each userid has at least a score and there is no upper limit for the number of scores of each userid.</p>
  
  <pre><code>+--------+---------+
  | userid | scores  |
  +--------+---------+
  | u1     | C,B,A   |
  | u2     | A       |
  | u3     | A,C     |
  +--------+---------+
  </code></pre>
  
  <p>I want to get the result by 'select ...' sql</p>
  
  <pre><code>+--------+---------+
  | userid | score   |
  +--------+---------+
  | u1     | C       |
  | u1     | B       |
  | u1     | A       |
  | u2     | A       |
  | u3     | A       |
  | u3     | C       |
  +--------+---------+
  </code></pre>
  
  <p>In hive, lateral view explode(split(score,',')) may realize the requirement,</p>
  
  <pre><code>select userid, score from my_table lateral view explode(split(scores,',')) scores as score;
  </code></pre>
  
  <p>However impala does not support explode.   </p>
  
  <p>Is there an alternative way to realize it in impala?</p>