title: How to join multiple select statements in SQL
body: <p>I would like to know how a query of this form could be improved on in terms of 1) Style and 2) Performance. In case it matters, I'm using Oracle as my database. Also, the purpose of this query is to select all records from the first select statement that do not have a corresponding record in the set of records to the right of the LEFT JOIN. The records from ColumnA are not necessarily unique in any of the tables.</p>
  
  <pre><code>select ColumnA
  from
      (Select ColumnA
      from Table1)
  left join
      ((select ColumnA, ColumnB
      from Table2)
      union
      (select ColumnA, ColumnB
      from Table3))
  using (ColumnA)
  where ColumnB is null
  group by ColumnA;
  </code></pre>
  
  <p>Thank you.</p>