title: SQL Inner Join for calculated column
body: <p>I would like to set sales year to date to automatically equal the sum of invoice headers where the account codes equal.</p>
  
  <pre><code>ALTER TABLE DEBTORS_MASTER  
      ALTER COLUMN Sales_Year_To_Date AS 
            (SELECT SUM (INVOICE_HEADER.Total_Sell_Amount_Excl_VAT)
             FROM DEBTORS_MASTER
             INNER JOIN DEBTORS_MASTER ON INVOICE_HEADER.Account_Code = DEBTORS_MASTER.ACCOUNT_CODE);
  </code></pre>
  
  <p>I get this error </p>
  
  <blockquote>
    <p>Incorrect syntax near the keyword 'AS'</p>
  </blockquote>