title: join query error on two super tables of TDengine
body: <p>I want to join query two super tables of TDengine database.</p>
  <p>The super table's schema is here:
  s1 (ts timestamp, v int) tags (t bindary(64))
  s2 (ts timestamp, v int) tags (t bindary(64))</p>
  <pre><code>taos&gt; select * from s1, s2 and s1.ts = s2.ts
  
  DB error: invalid SQL: super table join requires tags column
  
  taos&gt; select * from s1, s2 and s1.ts = s2.ts where s1.tag = s2.tag and s1.ts = s2.ts
  
  DB error: syntax error near &quot;tag = s2.tag;&quot; 
  </code></pre>
  <p>Could someone help to point me how I can do that?</p>