title: How to reference a custom field in SQL
body: <p>I am using mssql and am having trouble using a subquery. The real query is quite complicated, but it has the same structure as this:</p>
  
  <pre><code>select 
    customerName, 
    customerId,
    (
      select count(*) 
      from Purchases 
      where Purchases.customerId=customerData.customerId
    ) as numberTransactions
  from customerData
  </code></pre>
  
  <p>And what I want to do is order the table by the number of transactions, but when I use</p>
  
  <pre><code>order by numberTransactions
  </code></pre>
  
  <p>It tells me there is no such field. Is it possible to do this? Should I be using some sort of special keyword, such as <code>this</code>, or <code>self</code>?</p>