title: sum is not working after taking union multiple column in knexjs mysql
body: <p>I need a sum of 4 column ,I am doing that by first taking union then running sum</p>
  
  <pre><code>  db.select("gen-acute-sales-before-returns").table('sales')
    .union(function() {
      this.select("gen-acute-sales-after-returns").table('sales')
    })
    .union(function(){
      this.select("gen-chronic-sales-before-returns").table('sales')
    })
    .union(function(){
      this.select("gen-chronic-sales-after-returns").table('sales')
    })
    .sum({totalSales:"gen-acute-sales-before-returns"})
    .then(result =&gt; res.json(result))
  </code></pre>
  
  <p>Problem is that I am getting a union table but <strong>sum</strong> is not working</p>